Everything is silly, and consensus reality on these kind of things is just a glorified Reddit thread IRL. There's at least four plausible metrics. Everest is tallest from the local mean sea level (the smoothed gravitational equipotential—what a stationary water surface hugs); McKinley-Denali from its local terrain base; Mauna Kea from the local terrain base inclusive of underwater terrain; and Chimborazo, in equatorial Ecuador (it's Ecuador because it's equatorial), as measured from this planet's center-of-mass (the planet bulges out approaching the equator because of its spinning—"oblateness").

Like a Reddit thread, it's best not to argue too much with what the hive-mind decides. People literally died climbing what they believed to be the correct answer. Let them have their thing. :)

Then you’d be calling a whole continent a single mountain and it wouldn’t be a continuous slope in one direction.

I agree though that it’s a bit silly to measure Mauna Kea to the ocean floor.

Shout out to Chimborazo, where the summit is (likely) furthest from the center of the Earth. (I understand Huascarán is in contention, and don't know the latest details.)