I mostly agree with you. IDK what Mnuchin is thinking, but the way only I can see changes without legislation is if the current Regulations are "Legislative Regulations"
https://fclawlib.libguides.com/taxlawresearch/regs
Most Regulations are "interpretive". However, if the Regulations on cap gains are "legislative", the Treasury (not the President) could make changes that would have the full force and effect of law as if Congress had passed a law change.
Basically, "legislative Regulations" are where Congress gets lazy in writing the statute (law) and delegates it to the Secretary of Treasury to write out the details.
IDK what kind of regulations deal with cap gains and I'm too lazy to look them up, but I doubt they are legislative, and certainly not to the extent necessary for such a complicated re-write of existing tax law (too many other provisions likely impacted).
For those opposed to this proposed change - I don't think you should worry about this actually happening. It's just "noise".
Fern
It is kind of unbelievable that any administration would even float such an idea though, especially after their last tax plan turned out to be exactly the huge giveaway to rich people that it was predicted to be.