Alright, since I was in this round and was a part of the issue here Ill add my end.
As added by wue, the biggest threats of the round this far were swarmers, a tot, and a single vampire.
Swarmers were largely handled, by the one officer (PKP) and myself.
The traitor was largely inactive as I had both been informed of their actions, and actually happened upon their goal item. I informed them of such, and let them off with a warning. They then proceeded to spend most the round working in library to construct a vault.
The vampire ended up being my atmos tech, Pepper. I almost instantly found their lair with PKP while looking for remaining swarmers. We ended up starting to smash the room and got into a small scuffle before making a point to not make any issues for eachother. I do not believe Pepper did anything else the rest of the round. Nonetheless, this was not known to anyone really outside of PKP and myself.
This is important because no other threats or antags were visibly causing issues for the majority of the crew. Given all others were non-aggressive or dealt with, there was no reason to make combat mechs at that time.
This ends up coming into relevance as a pyro anomaly forms in armory where both Pepper and I respond. I find a Durand inside the armory, this is a surprise to me as I had not ordered for one. Apparently PKP tried to tell me of such, but I must have missed the radio message as I was messing with the TM’d gas rig in atmospherics most the round.
I should have, as acting captain, ordered the Durand to be smashed then and there. However, one way or another, I did not give that order. I suppose the thought did not cross my mind, but it should have which is my fault. For whatever reason, I just did not recognize the Durand in armory as an issue even though I had never ordered one to be built.
Jump forward some time, and xenomorphs are reported. I, as a result of getting reports of multiple xenos, order for the crew to arm, raise black alert, and order more mechs to be built. This ends up being several mechs as reported above, which are delivered to the scene of the fight. The result is the xenomorphs getting stomped relatively quickly.
I was personally only aware of the one in armory being constructed prior to my order. However, based on the report, it would seem most of these combat mechs were made before the xenos showed up.
I personally do not think the mechs were an issue if they were made after the order and the xenos were spotted. Xenomorphs are a well known round ending antag that win almost every time they roll. For the crew to have any realistic shot of survival, multiple mechs or excessive weaponry and coordination are required.
This does not, however, fix the issue of the mech(s) being made before the order and xenos.