While I'm only speculating, it could be quite possible that whoever may have wished to damage or destroy the AT&T building, seriously underestimated what these kind of buildings can take (essentially being disguised bunkers).

This certainly wasn't a small explosion, from the looks of it. But taking out a bunker .. unless you have access to weapons specially build for that purpose, good luck.

That AT&T still went down, may just demonstrated an apparent oversight in their emergency procedures for this site. I don't know the details, but having backup generators that rely any public infrastructure sounds like a pretty bad choice. That said, maybe those generators first burned through a decent supply of diesel fuel, only to later rely on (in this case absent) gas.

I have no idea how much diesel would be required to keep that site running for a prolonged period of time. Maybe such an amount could pose regulatory/safety issues on its own. Nonetheless, the sole purpose of emergency generators is to provide power that does not depend on any external factors (maybe aside from air/oxygen). So to me this story does sound at least a bit weird.

But somebody targeting that building and totally underestimating their chances, even if they may have still succeeded partially (if only by "luck") .. nah, that can not surprise me at all.