It may have been an idea just to inform people that the thread was going off target and maybe to stick to the point, rather than the first post from you being one to close it.
By closing the thread you don't give anyone the chance to "stick to the rules" because you have inhibited any further comments.
I appreciate a mod's job is not an easy one as you have plenty of dissenting voices to contend with, and heavy handed authority certainly is not going to help.
I have been on so many forums where the mods rule with iron fists, everyone is scared to break the rules, its like an electronic prison.
Those of you with BMW's will no doubt know bmwland, if you step out of line there they come down on you like a ton of bricks, that makes it an informative forum only where not much banter goes on.
It makes the place tidier to keep discussions in the right forum, and I do agree that it should have been conducted elsewhere, so why not move the thread rather than lock it?
I know you're new to this role, maybe this is part of the learning process, for you and for others. A warning is better than instant punishment.