Reading a bike review I came across this: "For seated climbing, I was really pleased with the zero-setback seatpost. Personal preference plays a part here, but this makes it easier to get your weight over the front wheel on steep climbs." Post setback is dictated by saddle position, is it not? What about KOPS or other measurements for proper saddle position? Am I missing something here?

The need for more setback is pretty much dictated by the size of the bike and how much room the rider needs to fit in the cockpit. If a bike was custom made there'd be no need for a setback post to accommodate the rider's dimensions because the seat tube would be at a more relaxed angle.

One reads all sorts of nonsense in bike mags. Ignore. Most people move back in the saddle if anything when climbing. So far I've been lucky enough never to climb a gradient where my front wheel came off the ground while seated.

You can have an aggressive forward position like time trial bikes, a position with lots of setback for more relaxed riding like touring bikes, or something in-between. I've experimented with both extremes on long climbs and favor the aggressive one. I live in the mountains and have zero-setback posts on all my bikes. For really long endurance rides on flat ground some fair amount of setback is probably the best option.

I like my fit better after I replaced the Zero setback post.

Unless ... you have the seatpost angle very steep to allow a rear wheel close to the BB, large tires and fenders and do not want to have to use a curved seatpost. My two ti customs were made with 74 and 75 seat angles. 72.5 would center a traditional seatpost in the middle of the rails of a correctly located seat for me. So I run seatposts with roughly 50mm setback. I find the rear wheel feeling very loose on downhill corners on bikes with longer chainstays. All the bikes that have felt really good have had short to very short chainstays. Short chainstays, 28c tires and fenders can be a challenge! Steep seat angles help a lot.

Ben

Slack frames = zero setback
Steep frames = lots of setback

I have a vintage road bike that I feel need a bit more seat setback. Will need to get me one of these offset seat post...

The reviewer presumably used the zero-setback seatpost to change their saddle position.

KOPS is a rule of thumb often used for initial fit of road bikes, but there's nothing fundamentally "proper" about it.

Saddle position needs to allow the rider to pedal, and posture them reasonably for the riding they want to do on the bike in question. Laid-back cruisers often have the saddle much farther back and lower than KOPS in order to position the rider upright and put all their weight on their butt; TT bikes often have the saddle farther forward than KOPS so that the rider can maintain an open angle between legs and torso when their posture is low and aero (and they use aerobars to make it easier for the upper body to support the resulting weight over the upper body).

If you're able to achieve the saddle position you like on a non-zero-setback seatpost, then you probably don't need a zero-setback seatpost.

Start at KOPS because it's a reasonable approximation. Then adjust AWAY from it until the desired fore-aft balance is achieved.