Not sure I agree here. The Spirits' famed frugality with their limited resources over long decades, successfully prosecute lengthy defensive campaigns in adherence with need to conserve resources, give up territory when necessary, forge limited alliances when beneficial, and heavy emphasis on high-quality training all speak to a generally good strategic grasp.
Exactly. We saw plenty of that both before the Absorption War, and afterwards in FM:U and WoR. Granted, they made some mistakes, but pretty much
all of the Clans have made poor tactical and strategic decisions throughout their existences. Even the Adders aren't exempt from this, as we saw with the resurgent Burrocks.
I think it's very easy to question that strategic ability considering the interference in the Burrock Absorption, and I used to be very critical of that decision, but now I look at it as the Spirits almost had to try. Their fiercely, deeply hated rivals about to be laid low by some other schmucks? If Karianna Schmitt didn't attempt to intervene on her terms, I think it likely she would expect somebody (potentially lots of somebodies) to come for her job and then intervene against the Adders and Burrocks. If she didn't, someone else would, in other words.
A few things here to add to your point.
First, as you said, the Spirits simply had to try. The Burrocks had spent almost the entirety of their mutual existence going out of their way to prey on the Spirits, and that's
after their failed attempt to get them Annihilated for having the audacity to suggest trying to find another way to deal with the Wolverines. FM:CC mentions that the feud had unbalanced both Clans, and we all saw just how bitter and hateful (and rightly so, IMO) the Spirits had become. Of
course they would jump on the chance to end the Burrocks for good.
On top of that, you have the Grand Council
assigning the Absorption to the Adders, which is technically against Clan law anyway, not giving the Spirits (or even the rest of the Clans) the opportunity to bid for the right. There's no way you can convince me that the Adders' nine-out-of-ten Galaxies used in the Absorption (which fits the Adders' conservative bidding practices and makes perfect sense for them) was the lowest bid on the table. The Spirits had a legitimate grievance here.
Also, the Spirits had no way to predict that the Burrocks and Adders would ally in order to repel them. That's simply not typical Clan thinking; they're very much a dog-eat-dog society, and it was absolutely unprecedented that the Burrocks would, by-and-large, submit willingly to the Adders instead of fighting for their right to exist. Before that point, that's just not something we've ever seen from the Clans, who we all watched sabotage tf out each other during REVIVAL.
Then there's the elephant in the room: the Adders themselves were in this very same position when the Jaguars were given the right to Absorb Clan Mongoose. Per WoK, they were outraged when the Jaguars were given the right of Absorption despite their own long-running and costly feud, and they launched pre-emptive attacks of their own. It's only because Jaguars gonna Jaguar that the same thing didn't happen to the Adders when
they decided to interfere in the Mongoose Absorption.