At the very least blaming Russian intelligence would have tied right in with all the other election related hacking allegations.
Also, I think you may overestimate just how embarrassing this is relatively. USG regularly announces that they got owned, is the NSA getting owned worse than OPR getting owned? Maybe not.
That's not how it works. There is no coordinated messaging campaign across agencies against adversarial nations.
It might seem that way based on media reports, but there isn't.
If the FBI believed that TSB was Russian intelligence, it is perhaps somewhat surprising that it's not mentioned in the Mueller report.
We also still don't know how Hal Martin was related to TSB, the timing of the messages sent from his twitter account to Kaspersky (who apparently reported him to the NSA!) does pretty directly tie him to the TSB.
And the Mueller report was specifically scoped down to not include anything outside of the 2016 presidential election. I don't know why you keep going back to that.