Is that because all the voices are equalin nature, since it was written for the harpsichord (and virginal and whatever)?
And in a fugue the main theme just happens to pop up every now then in the different registers - it would be polyphonic even if it didn't replicate the theme over and over?
Anyway, as long as it isn't a chorale etc. I can be quite sure it's polyphonic?
There really isn't time for a theory course -- and it wouldn't fit well in a forum anyway.
First, not all polyphonic music was written for keyboards (including, besides harpsichord, a LOT of other instruments, especially organ) -- a good bit of it is vocal, and another substantial chunk is for various instruments; sometimes in families and sometimes mixed.
Second, a fugue has a very particular construction; the main theme doesn't "just pop up"; it may appear in retrograde, or inverted, or augmented or diminished... etc. Indeed, it may be rather hard to tease the main theme out of the entire polyphonic structure. (I note that a canon, on the other hand, does repeat the main theme over and over; think that famous thing by Pachelbel).
Third, the voices do not have to all be equal. It is perfectly possible -- indeed not at all uncommon -- to have a piece in which one voice is carrying the main theme while a several other voices are carrying on more or less complex counterpoint around it. Most of the Bach chorales are like that.
The Bach chorales are all polyphonic.
Please... go learn some theory.