Nobody can decide whether, in English, we sit "in" or "on" a chair, so it seems to me both should be accepted. Currently only "in" is accepted for this sentence, although "in pavimento" means "on the floor." [Reported.]
I've never seen it phrased like that. Is "sitting in a chair" a normal thing to say?
Thinking about it more, I could see saying sitting "in" a chair if it had an extremely-high back and sides, so it was sort-of enclosing you. But I'd always sit "on" a regular chair or a bench.
In/on is a hard distinction for some non-native English speakers; even native speakers have trouble explaining how we make the choice (for instance, we say "on the bus" even though we're inside it, possibly because historically, omnibuses were open, and the phrase stuck after they got sides, roofs, and motors replaced the horses; that also applies to other public transit, like trains, planes, ships, etc. If you say "in the bus" you're making a special emphasis, e.g. "three victims are still in the bus.").