From memory, Elon has made it clear repeatedly that non Tesla vehicles would be allowed to use Superchargers, as long as appropriate costs were contributed. Clearly any arrangement making this happen can only really occur at the manufacturer level.
In fact, it appears that this has already happened for one smaller manufacturer [0].
So, while it may be likely that regulation is necessary to ensure that a fragmented charging landscape doesn't permanently settle into the US market, "only Tesla's can use Supercharges" is provably false.
[0] https://www.electrive.com/2020/12/22/tesla-superchargers-alr...