Yeah, this is wildly illegal. Actually more obviously illegally, possibly even more obviously unethical, than the Ukraine stuff or the emoluments clause stuff. Speaking of Secretaries of State, I could make a "BUT HER EMAILS" jibe too. But that would be lazy and a forced meme at this stage.
It is not illegal for a Cabinet member to speak at a convention.
It does violate State Department policy for the Secretary of State to do so.
Violating policy is not violating the law.
Violating the Hatch Act is, though. They won't be admonished by this administration, but it would still constitute a violation of law.