Bombards have a 2x bonus to everything in my fleet.
Well, that was part of your problem...you used a strong concentration of units that are weak to a unit type the AI got and was packing its special forces with (as for whatever reason, it seems like your AI decided to focus on long range stuff for its SF, hence the strong favor for zombards and missile frigates).
However, that does bring up a good point. Their base damage and raw range are enough that they don't really need bonuses. Giving them bonuses makes them near uncounterable in some cases. This is a very similar issue that the hunter/killers have with their 2x structural, 2x ultra-heavy, and 2x heavy bonuses (the H/K has so much DPS, is doesn't need these. With them, they start feeling really cheap and uncounterable, in a bad, unfun way)
That, and it may be a good idea to tighten up the special forces strength cap, so it can't bypass the purpose of the special forces ship count cap quite so easily by often choosing low cap ships. As you can see in this case, the special forces strength cap failed to do its job (especially because there are around the same number of zombards as missile frigates, which if the special froces strength cap was in a sane place, the ship cap cost scaling would of dominated its choices, as it would run out of "strength room" to pack more zombards in, and it would have to fill in the remaining strength with missile frigates or something else).
Wait, why was the special forces on your planet? Did you "wall off" parts of the galaxy that still had SF posts on both sides, so it had to route through your planet? Was that actually threat fleet?