these Macs can't go below Tahoe. People on Mac Rumours were complaining about M5 MacBooks unable to install Sequoia, so it's safe to assume Pro/Max chips will be the same.
This. You can’t downgrade below the version the device ships with (a forked build of the current version at time of mass production)
I've also returned Apple products multiple times, once (recently) without the packaging, and once several days past the return window. They refunded me every time, no questions asked.
This makes me wonder if it's part of their training?
Can be a little annoying (an employee actively tried to downsell my partner, even though they knew what they wanted), but overall it’s a nice practice.