If they had the ability to create the anomaly from one of their structures at will, then they would've done that earlier in the month-long conflict in this system.
What, and risk not having all the human reinforcements be committed yet so that they could wipe them all out at once?
There is also another specific advantage to this timing for the Thargoids - if they'd fired it first, it defeats but doesn't
discredit both Salvation/Azimuth and their backers like Hudson, Mahon, etc. With this timing, it does both, and very publicly too.
(I'd been expecting them to let it "work" and then come back later; I was expecting "a month later" because I'd not picked up on how effective "a minute later" would be. Nicely done...)
I wasn't playing during the early days of the Thargoid story arc, but from what I can casually glean it seems the Thargoids have lost battle after battle and have lost territory after territory after the Humans found Guardian paleotech and adapted it.
The Pleiades nebula, the Witch Head nebula, and the Coalsack nebula were all lost because Humans are too good in creating very effective Guardian-Human hybrid weaponry.
The unclear thing here is how much the Thargoids have actually been
trying to hold that territory in the short term, though.
They've retained the ability - even before today - to strike and damage stations with impunity, and clearly could have destroyed the stations they disabled had they wanted to. But rather than following up those attacks, they've hit a couple of systems and pulled back again.
There's no obvious reason that they didn't just continue to hit system after system - they easily had the capacity to damage infrastructure faster than we could repair it, and hit-and-run tactics would have let them preserve the vast majority of their ships in the process. What use is fancy weapons technology if you eventually get cut off from your resupply. I suspect the Thargoids may be quite happy to have some arms-length coexistence with humanity, if we're happy to have the same.
Whether that "making a point that goes over humans heads" approach will be continued after today, who knows. They've made a pretty effective point today, but certainly might want to reinforce it further. A counterstrike against high-value human assets might reinforce discontent with the current superpower leadership.