Oh look, it's a plectrum piano!

[RJVB]

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[AllanAllen]

Yeah, a Harpsichord.

[Mick-7]

So which Baroque composer did Joseph Kosma plagiarize?

[RJVB]

Just as many as all the later composers JSB plagiarised - and the actual composition comes from a ballet piece that was inspired by a piece by Jules Massenet. The music used for French chansons is usually a fusion of different styles but rarely evokes baroque music directly.

According to the comments the performer has a recording out on which she performs pieces (hopefully not this one alone) in the style of various composers but this was supposedly recorded on the fly.

I think the point here was that a harpsichord can sound pretty jazzy even in music that isn't officially jazz. (Something I've begun to suspect listening to a Bach recording by Christophe Rousset, but never really got confirmation of.)