My question here is totally different than you are saying. I'm looking for a way to check who locked a thread. Whether it is locked by moderator or locked by the thread starter/OP themself.
If you understand the point of my post, then you should already know that you won't find out unless you ask the OP or a moderator about who locked the thread. Additionally you also mentioned statistics on thread lock actions by moderators, but so far those statistics are not available. Anyway, mprep has answered your question which should more than suffice, right?
Do you think it is important to have statistics like that?
If it's your thread, try unlocking it. If it doesn't allow you then a mod or an admin locked it. If it isn't yours, then AFAIK there is no way to tell (without obviously asking the author to try and unlock it). Not sure whether that's intentional or just how SMF 1.x works.