Giants?

[The Silver-shod Muse]

Except that Bilbo knew what trolls were.

Yes, but there are very many different kinds and sizes of trolls (i.e. cave, mountain, etc.) and if I remember correctly, the weather was very stormy and visibility was probably not favorable. I'm sure that if a large, indeterminate something were hurtling boulders at you in the middle of a raging storm, you might be inclined to feel that the situation is worse than it really is, if that is possible, hence Bilbo calling them "giants" instead of trolls.

Besides this, trolls were already used in Bilbo's little "story", and perhaps, as you said burrahobbit, "what wasn't cool he made cool."