Well, the information on how many participants and what quantity of actions were carried out for those CGs is publicly available through Inara and other sources - why not have a go for the next one? (Seriously, it's good fun)
I'm not sure it'd be much help in this case: there have only been two Thargoid Combat Bond CGs in the current plot, and both were a while ago (specifically, before the rebalance in bond payouts for Thargoids!)
- Coalsack, 16 November 3306: 80B bonds in four days
- Witch Head, 21 October 3306: 80B bonds in six days
Prior to that the last one was on 17 March 3304: 3.5B bonds in three days - but that of course was before a lot of the larger Interceptors, or most of the anti-Thargoid weapons, were available, so hardly comparable at all.
So this one has done - for the three CGs combined - 433B bonds in five days. I'm not sure that was necessarily predictable given a baseline of three CGs, all of which were in very different circumstances.
FD know how popular AX combat is, how many bonds are gained per hour in normal play and how many bonds have been stored by players. In addition they can see how popular the CG would have been if they ever read the forum so it's not a great leap to have an accurate target.
I do like an extended target as it can sometimes mess up the payment for tiers and they can become very lucrative which is especially handy early on. I am however, not in the slightest interested on doing a CG involving AX combat.