My sincerest apologies to diabetics everywhere.
First of all, fillies are young female ponies. Sombra was a colt.
Second, wouldn't it be easier if the umbrum disguised either themselves or each other and pulled a Trojan Horse on the Crystal Empire?
Third, this is exactly how the comics do King Sombra wrong. This is the kind of info on Sombra we were NEVER supposed to be given. We were supposed to know as little about him as possible in order to dread him. But the only reason that failed is because Meghan McCarthy strained that attempt to a point that Sombra basically wasn't even written. And since he had next to no character to begin with, it's hard to care about where he came from, or why he was evil.
Thankfully, these are the comics; it's McCarthy who decides everything about King Sombra, and it might be just me, but I honestly doubt she'd see this resonating with the Sauron reference and Darkness Incarnate she tried to pull off.