I think $tc_db->qstr is safe enough because it's used ten lines later the exactly same way.
This mod allows to post more than one message in a thread without re-entering captcha/recaptcha. The thread must exist and must have at least one reply made few minutes ago from the same IP (OP doesn't count because it's not a reply). So one need to type captcha once for multiple quick replies and to type captcha twice if he have to create a new thread first.
However, this mod does not allow to create new threads without captcha and to flood a thread from different IPs (proxy/tor). The worst thing it allows is to do is to flood the thread to auto-sage limit and beyond, but all those posts can be deleted with one click because they're forced to have the same IP.
Also, it does not affect min. post time limit ("7" in the example). But it doesn't work because of a bug, BTW. KU_NEWTHREADDELAY works via $tc_db->GetOne and it's OK. My mod works via $tc_db->GetOne and it's OK, too. KU_REPLYDELAY works different way... wait, I can't say it WORKS different way because it DOESN'T WORK. I don't really care. Just warning: don't blame my mod, KU_REPLYDELAY does not work already. If it do, both time limits will work: user must not post with flooding rate but can bypass captcha if answers quickly.
Of course, captcha field is shown either way. We can't predict future. But user should be aware it's no need to fill it if he already posted to the thread minute ago.