> Does it touch on why people initially became so opinionated about serif/sans readability?

That's the default state of all questions. It doesn't need to be explained.

Why do you think people had opinions on whether Pluto should be called a "planet"?

On both cases it is based on some evidence even if they are completely different (one is a question of definition, another of measurement and observation): for Pluto, it is a round lump of rock going around the Sun on it's own separate orbit; for serif vs non-serif, argument is that serifs help with line tracking for eyes depending on the line spacing and line length.

For a meta-study finding a different result, it'd be great to qualify how was the previous research wrong so we learn something from it.

I've marked as something to pick up as I am very curious.