Sanded, stained, and 2 coats of finish for all. Then cabinet install. Third at the almost end of the project. Then toekicks and prepainted baseboards installed and touched up. Yes, that is harder. Yes, that is the best route.

The flooring can be sanded afterwards. The finishers need to becareful not to damage the cabinets, that's all.

It's easier and faster beforehand.

No right answer here, if done correctly you would never know at the end which way it was done. I have done it both ways depending on scheduling.

"GC is telling me they do not need to be sanded before."

The GC is correct. They do not NEED to be sanded before.

Some contractors like to sand and finish before cabinets but that comes with risks, considering how many subs will be working over a finished floor. Doing all but the last coat before the cabinets is also very risky. Floor people are often expected to fix whatever damage may have occurred, possibly without extra compensation. If a fix doesn't look great, the floor may have to be done over, again, possibly without compensation. Then there's the issue of finish adhesion if someone used an unapproved product to clean the floor.

To sidestep this we often rough sand the perimeter for baseboard and where the cabinet boxes sit to make the base and cabinets sit flat on the floor and eliminate one sanding next to the cabinetry and trim.

I do not like big, heavy, hard to control, sanding machines bumping around near XXK worth of custom cabinets. Or XK worth of stock cabinets. The cabinets cost more than the floors, and are harder to fix impact damage.

Floors are pretty easy to protect, with the right products. Cardboard, blue foam roll, and another layer of cardboard, all taped up. (Save those cabinet boxes!) Cabinets are not easy to wrap in moving blankets once they are installed.

Yes, we’ve done floor finishing after with no issues, depending on the schedule. But I only trust a few specific contractors for that. And they are slow to earn that trust. I’ve run into far too many GC’s who have subs that really aren't specialists, and who shouldn't touch the project that they are handed. YMMV

So yes, absolutely sand when there are fewer obstacles in the way!