The previous theory was wrong because someone made it up on the basis of nothing. This is a very common event.

It is precisely analogous to the "dispute" over Pluto,¹ where the only argument was "I was taught in school that Pluto is a planet, which means it is true that Pluto is a planet". That is also the only argument for differential readability of serif vs non-serif fonts. It shouldn't surprise you that it turned out to be wrong.

¹ (The conclusions are not analogous - "Pluto is a planet" is not capable of being true or false when the definition of "planet" is up for debate. "Serifed fonts are more readable [under condition X]" is capable of being true or false. But the arguments are identical.)