I've had a look at some sellae. Early ones seem to have a flat surface to sit on, with no back support (sella curulis) but later developments provided a strip of supporting material across the back, Either way, they were nor "armchairs" in the modern sense, so I would say "sit on" fits better than "sit in". Even now, you would sit in an armchair but sit on a chair, or even sit on a sofa. To "sit in" needs a sense of enclosure, I think.
Both could be correct, depending on the type of chair.
I think the general word would be more "seat" than "chair" to describe any kind of "chairs", deep one, like armchairs, sofa, benches, etc... But it seems that "chair" is also used as a general word (umbrella term).
Sit in —
An armchair, a beanbag
Sit on —
A dining chair, a stool, a bench
I think they should select "on" if it's really a chair, not the umbrella term, in their suggested correction.
I have no idea if "sella" was an umbrella term in Latin or described a specific kind of chair, the common one.
But I know that "sella" gave "sellette" in French, and also "selles" meaning stools (oh, same thing in English with "stool", funny), meaning feces.
The original meaning was "aller à la selle" = go to the "selle" = going to sit on the thing you know, described as a chair (but a stool in English, stool was originally a chair, before the word "chair" was borrowed to French "chair", because the German for chair is still "Stuhl")
It also gave the word "selle" in French, meaning a saddle. Because we sit on it.
And in Spanish, "sella" became "silla".