That is false. To quote article 2: the president "shall take Care that the Laws be faithfully executed" i.e. the laws passed by congress. If congress sets some rules that the SEC enforces, the president cannot change those rules (laws).
Yes, this is the way that the Constitution is supposed to be followed. However, the current president has realized that only he can enforce the laws, so if he breaks, invents, reinterprets, or ignores the laws, there is no one else that can punish him. This is the way government functions in banana republics, which is what the US unfortunately is being transformed into.
The only constitutional mechanism for reigning in a wayward President is impeachment. Perhaps in the 70's with Nixon, congressmen might think about the good of the country, but in our times, it's all about politics and preserving the power of one's party, so impeachment in practically impossible. (Or more pedantically, impeachment is easy, but conviction in the Senate is practically impossible.)