Will this stand hold up a 100g+ tank?

[Ulu]

. . . Sure the framing is particle board, but theres quite a bit of support . . .

You cannot assume that.
The frame & ledger construction is not visible in the photos.

It's almost certainly already overloaded.

I just had all new 2cm stone counters installed, with new cabinets, throughout the house.
The construction is not even close to a proper aquarium stand.

But reinforcement is certainly an option.