The Mask has been known to fail, even spectacularly, before.
The only evidence they had against Lucas Penrose was that he was a "Blank" someone who didn't exist before joining the Order. And that he bolted when he was being brought in to have that little detail discussed. He doesn't actually ever admit to it either, though he does suggest that bombs are "crude."
There's no evidence that it was the WoB, and the only reason they're blamed in Prince of Havoc is because they're the only group that people think could slip an agent into ComStar.
Mind you, just a page or two earlier, Victor and all are discussing a group of Nekakami who managed to not only slip 1 agent into ComStar, but *4*. So much for the Word of Blake being the only group able to infiltrate ComStar.
So pretty much the only IC "proof" that the WoB was behind it can be ignored because the extended Superfriends are idiots.
The only OOC Proof is that in the ComStar Field Manual, the Precentor Martial of the WoB says "Not the Glengarry Black Label" with a small smile. Which is pretty much stretching it.
On the other hand, we know that pretty soon Sun Tzu would be retaking the St.Ives Compact. Having the Morgan as the head of the Capellan March would be alot worse than having his son or wife. We know that Sun Tzu's view of the Star League was "How much can I get out of this," meaning I can believe he was perfectly willing to get rid of Morgan, risking Serpent. After all, if Serpent fails, or takes longer, what has he lost? A regiment? On the other hand, he forces his opponents to dedicate more to the eradication of the Smoke Jaguars. And if the eradication fails, its not like it really affects him much. His realm isn't under attack by them.