It's a channel strip, always has been.
To my knowledge, it's the first full channel strip for the 500 series.
Deleting the preamp would be very little savings actually, considering it really is a very decent preamp, just call it a bonus...or just run it line level...!
Everything the P1 (and now the P501) does is clean, from the pre / DI to the compressor / limiter.
War