I think you confused yourself.

You asserted that you felt a frame was different simply by sitting on it.

I replied that you most certainly did not, all you felt was the seat and a difference in stack/reach.

Now you're saying that I said you can't tell a difference in stack/reach when sitting on the bike.

But that's what I said initially to counter your utterly ridiculous assertion that you could tell the difference in frames.

Slow down your reading a bit before you get even more confused.

Zero spacers on the bike. I didn't realize that bikes came with spacers and that I could use those to modify the stack the height. I learned something new today. I don't need a tall stack endurance bike, I can easily touch my toes at 6'2", flexibility is not an issue.

Let's dial back the snark.

Everyone likes to say there wil be zero difference between the frames... but I personally would want to ride a bike for a "typical" ride, or at least for10-20 miles at a minimum, to observe and confirm that expectation with my own butt before dropping $10k.

Personally, I have a 2008 sworks tarmac SL2 team (bought frameset new and built w RED 10 and dv46 wheels, in 2017 repaired paint cracks, repainted, rebuilt wiith clx40 wheels and etap 11 wifli.
I rode (for a very short period) a sworks SL3.
Whlle the SL2 was "broken" I built a sworks SL4 frameset up with the RED10 parts.
I have a modern masi evoluzione MSR team frame (currently built up with the same parts as the SL2 and boyd 24 wheels).

IN MY EXPERIENCE:
Moving wheel sets between the bikes does not change the ride of the bikes, but the deeper wheels have an interesting acoustic effect directing sound to the sides and echoing it off hard surfaces to either side. Interesting, but the wheels ride pretty similar in terms of not changing the feel of the framesets.

But the framesets themselves definitely feel different in meaningful ways.

When i've been riding the masi for a while and finally jump back on the SL2, I still, even after owning it for 12 years, get the initial feeling that the rear tire is low. This is because the rear of the bike is softer and more giving over road irregularities. This is a good thing, as it's the carbon design doing exactly what it was designed to do.
However, the SL3 was a bone crusher. I heard (anecdotally) that it was because Tom Boonen was a beast and demanded an extremely stiff frame and got his wish. But for me, the SL2 was clearly a better frame for me on long rides (60-100 miles).
The SL4 was interesting because I got the soft feeling at both the front and the rear. Specialized had figured out the carbon design to let the front and back both flex vertically. The SL4 was the best riding bike I have ever ridden.
The masi is.... carbon. It's a good bike. But the SL2 and SL4 are both clearly products of extremely focused, expensive engineering.
I have not ridden an SL5, SL6 or SL7. I paid $2000 for the SL2 brand new ordered for me , and honestly I'm not in the mood to pay $5500 for a frameset that for all I know could ride like the SL3 bone crusher or the masi (whcih I can buy for $400 on ebay).

I would suggest you DEFINITELY want to test ride a frameset (ideally adjusted and with a stem somewhat close to your fit) on a decent ride that will mirror your typical use. Look for handling characteristics. hands off? Crosstwinds? Handling? Compliance? Stiffness sprinting or chasing on a break? Comfort? These are the things you want to evaluate.

No spacers on a bike on the sales floor? I find that hard to believe. Was the steerer trimmed, too? If you didn't know that bikes came with spacers, I'm thinking that you might be an unreliable narrator.

Lol. You haven't been the recipient of snark for no reason. You're the one asking questions about things you don't understand, yet you've been snippy. Dial it back, indeed - we'll follow your lead.

If you live in flatland, grams, even almost a full pounds worth of them, are 100% irrelevant, and you should (probably) be looking at a Venge and not a Tarmac. Weight only matters going uphill or maybe racing and even then only going uphill. Aero matters everywhere else, including going downhill. Of course aero also kinda depends on riding position, and bolt upright is more of a casual riding position than a performance minded one.

It's not about flexibility, but core endurance. When you lean forward you hold yourself up with your core muscles, predominantly your lower back. How long can you hold a superman position? 5 min? 30 min? 2 hours? 7 hours? Yeah. It sounds easy, until you actually have to do it. Until you've developed core endurance lowering the bars will only put more weight on them when your core is incapable of holding you up. This is why endurance bikes have a higher stack height, not because endurance riders somehow lack flexibility!

Having too much weight on the hands is caused by the saddle being too far foward. With enough setback, there will not be much weight on your hands, so you're not using core strength to hold yourself up.

With the saddle back, you're still using core strength, you just have a balance point to cantilever against. Saddle too far forward puts your balance point too far forward and you're left with no choice but to support with your arms.