Isn't the Marathon a bombproof tire with a really stiff/rigid ride? I don't see the point of running it tubeless.

The Marathon Supreme is one of the lightest in the Marathon series. A tubeless 40mm Supreme might be lighter than a 38mm Compass Barlow Pass w/tube (but the Compass will probably be smoother and faster).

Plus tubeless has advantages other than lightness and smooth ride.

That's the "Marathon Plus". Rides fine with the right tire pressure, and as close to flat-proof as I've seen. A great commuting tire.

No.
"Marathon" is a product line, not a single product.
And the Supreme is very different from the others.
When it was introduced, Schwalbe made a point in the marketing that it was using the same soft compound as their racing tires. (Haven't heard that from them more recently, so perhaps the racing tires moved on to other compounds.)

I've been using 32mm Supremes for years on my steel touring/winter bike.
They have excellent grip in the wet, and, (amazing), also on snow!
The only thing they have ever lost grip on is glare ice.

...are you contacting Schwalbe North America link ?
I have always found them to be very responsive and helpful.

I even waranteed a badly laid up marathon to them once, using only photos of the defect.