Is it really possible that there's such an "easy" route, which is graded as 6b?
Yes, it seems to be possible, cause I just climbed one in Tapanila climbing centre. It was a corner type of 6b with more or less handle holds and few slopers. So the holds were actually quite comfortable and the route wasn't really physically demanding. Although, the moves were quite long and maybe some how more 6b'ish, so that might be the reason why it's 6b.