Even with secure boot disabled you can't install Linux on the internal SSD. Installing Linux on a Mac has already been very flaky for the last few years, but now is impossible.
The likely problem is a lack of driver support for using the T2 as an SSD controller. I don’t think, based on Apple’s white paper, that they did anything to explicitly block Linux from accessing the internal SSD - it just needs to go through the T2 for that.
Hopefully someone is working on the necessary driver support - these laptops are still very new so maybe nobody has gotten around to it yet.
Apple is actively blocking unsigned software from accessing the internal storage as a security measure and providing no means to add allowed keys. Its possible there is a defect in this security that could be exploited but it would be explicitly a bug and would be liable to be patched in the next version of the software. You have completely misread the situation. This is apple taking over your machine while still expecting you to pay for it.
So it doesn't stop you in a way a game console might, but you lose some features of the hardware by doing so.