I mean, he also did know the President was on his way out physically based on his “He looks like death,” comment. He just didn’t know it was cancer. If that gets out that Vice President Carter (if he had stubbornly stayed on) had suspiscions of the Health of President Humphrey and didn’t invoke the 25th in a hearing in front of Congress, his re-election in 1980 has about a snowball’s chance in the Sahara.
Good point.
On the other hand, if Carter invokes the 25th HHH just has to say "No, I'm fine" and the 25th doesn't apply. Carter would then need the backing of the Cabinet and ultimately Congress. It would look like he was trying to usurp Humphrey.