True. I wondered about that myself. My positioning there was based on the fact that A) Brook hadn't joined the crew when the "weak" trio went to Thriller Bark, and 2) Chopper actually had quite a few battles under his belt by that point. Brook's track record pre-timeskip was finishing off the Spider Monkey that Nico Robin and Franky fought, losing against Ryuuma twice, and putting some people to sleep. Chopper at least could fall back on Monster Point.
Well, i dont think loosing against Ryuuma should be held against Brook, as the guy were tough enough to give Zorro a run for his money.

And as for chopper falling back on monster point, then i dont really think that should be considered as a power boost either, since doing so would have send him on a senseless and selfdestructive rampage before the power boost.