Another potentially interesting theory, that would have more impact if the blond "family" are in fact UB in human form, is the connection between Gladion and Type: Null. If Gladion is the Expansion UB, perhaps one reason he chose Type: Null as a partner is because of a sense of kindred spirit. What I mean by that is, Gladion and Type: Null are both beings of immense power and strength, however, for what ever reason, that power is being restrained. For Gladion, it would be his human form, for Type: Null, it would be that mask.
Also, for those wondering how Gladion could be the Expansion UB or how he could be connected to it like Lillie and Lusamine are connected to "their" UB or how people are able to see a connection between the two, think about this. Gladion and the Expansion UB share a color scheme. They both have black spikes on their shoulders, Gladion's name is based off of Gladiolus or "sword lily" and the Expansion UB has a sword-like nose. Gladion's description on the official website labels him as the Team Skull Enforcer and talks about how he values the strength in Pokémon, something Expansion UB embodies.
Not to mention, it would seem kind of weird for only 2 out of 3 of the blond trio to be connected to the UB, especially when they all seem to, intentionally, be on different sides of the playing field (Gladion=Team Skull, Lusamine=Aether, Lillie=nuetral?). Definitely seems to be a connection between Lillie, Lusamine, and the UB, why would Gladion be left out? Of course, this is all still speculation, but this is just so folks know people aren't pulling the Gladion/UB connection out of thin air.