It's balance thing, propably. If Shield Bestowal would upgrade with standard shield research, it would make destroying Advent garrisons a major pain in the ass. Besides, Shield Bestowal isn't a same kind of shield like the ones on their ships, so there is a reaon why it doesn't upgrade. If Shield Bestowal was to be upgradeable, there would have to be a separate lvl 6 or 7 research item in the defense tree for that.

Advent shield research also increases their idle mitigation, as does their culture, which means they take less damage than other races. If these would also effect buildings, even small garrisons would prove to be a though nut to crack. The Shield Bestowal ability is meant to be a small buff to their defenses, like the TEC hangar flak cannons, and Vasari's Phasic Trap.

I fixed it, someone clearly didn't know what they were doing when they wrote the article.