@Luuklag The question not only asks about how they were able to put a bounty on a comment-locked question, but also about why they were unable to award the bounty to a (completely) locked answer. The duplicate is applicable here. In addition,
meta.stackexchange.com/q/19756 is also applicable here since that's the FAQ that explains comment locks and how they permit other interactions (section
Why can't I comment on specific posts even if I have enough reputation to comment?)