Yes, locking is a final state for a question. The purpose of a lock is to clearly identify that a question is not a good fit for the site, but it really shouldn't be deleted because it's important in some way. It may be because it has a high number of views (indicating that it may be referenced in other questions or on other sites, meaning that deleting it would cause people without sufficient reputation to receive an error message). It may be because it has valuable information (although the question isn't a good fit, there are valuable answers that may be helpful, but we also want to discourage others from asking similar questions in the future). Or it may be because the scope of the site has changed over time and we don't want to remove contributions that, at one time, were acceptable and of a high quality.
It can be undone by a moderator, if there's a good reason to. If you think that there is a locked question and it shouldn't be locked, feel free to start a question here on Meta, indicating the question and reasons for unlocking it. The community can decide and a moderator can undo the lock. In the case of the question that you linked to, I don't think it should be unlocked. Career and education advice are explicitly off-topic. However, it has had over 30000 views over 3 years and two high quality answers.
Note that even if commenting was allowed on locked posts, we still wouldn't want editing to happen. We don't want to bump these old, locked posts to the homepage since they aren't good questions. Locking just enables them to be found via searching (including Google) and external sites to prevent knowledge from being lost.
There is one exception to locking as described above, and that's the "wiki answer" lock. It reduces a question to one community wiki answer that has a low reputation barrier to edit and prevents multiple answers from being posted. I believe commenting is still allowed on these posts, but I'd have to find one to check.