Next is not newbie members, but high-ranking members, who did not notice how old that thread is, not only four months but maybe four years or older, was pumped by a newbie. They joined those discussions and did not know that they just replied to a non sense post from newbie.
I don't know how newbies can find threads that already are inactive some years, and bump it.
A random search can bring out outdated posts sometimes and if you don't pay attention to the date you won't notice it is an old thread. I have experience it before numerous times but there's no need to respond to such posts unless it's very necessary. By the way, the forum use to issue a warning note when you attempt to respond to old posts reminding you that there has been no reply on this thread for a couple of months now and whether you would like to continue or better still create a new thread. I assume that some people don't pay attention to notice the warning before posting on the very old thread.
OP can lock it and should do it but if OP did not lock it, months later, it was bumped by newbies, it's hard to say the spam is because of OP.
Other posters who join those discussions have responsibility too by did not read OP, but only reply to a newbie's post to reactive an inactive thread.
I can't blame the Op or people who joined the thread lately. We normally come across old but useful thread that engaging in it might take it to someone else notice whom might benefit from it. I prefer to keep the thread moving as far as people are not derailing from point of discussion than seeing a new thread that looks similar and will get almost the same responses, ideas and opinions just like in the old thread. A close observation in this particular board is typical example of board with a repeated thread. Most questions or threads here can be a reply on a similar thread and someone will probably provide an answer to the question. There's no point in creating new thread if there is a thread it can fit in as a reply but almost everyone of us is guilty of this.