The Paris-based International Astronomy Union (IAU) is not really astronomy’s official governing body any more than NASA is. They often claim to be but they are a few hundred astronomers. They do not determine official designations any more than a few scholars with expert in CRISPR at a National Academy can tell FDA to stop treating harmless gene editing in plant varietals as a new drug in the regulatory scheme.
Pluto was demoted by a vote of 237 people, not the science community. It was widely derided in jokes at the time.
There is no official governing body for astronomy, that is why if you want to spend $1 and have an asteroid named after your kid, you can. IAU will say such a name is not official, but their arbitrary ruling on Pluto, and the fact that 98% of astronomers had nothing to do with it, show how little clout they have. There is also no official government body of cancer, which is why you can still buy bacon and drink hot tea even though IARC statisticians (also in France) did their best to try and link those to cancer.
Pluto could be one of 9 planets, or one of 13 - the IAU botched their effort to be meaningful that badly when they created a new arbitrary definition to replace an old one.
Bridenstine should not say 'that is how I learned it' as a rationale, people my age were also duped into believing "nuclear winter" was real and that NRDC was a legitimate environmental group. But while he's not right to say Pluto is a planet, he is not wrong either.
IAU has its own internal politics. Many inside know their own new definition is batty, but they do not want to stand up to insiders who will shout them down. Yet until they come up with a legitimate definition of planet, no one is going to rush to agree with them.