I would imagine they use mods,because I had also heard that the normal max was Charisma + 10 and while Charisma can be increased through gear and meds and such that would still not be ~such~ a huge number.
It may also be possible to shuttle settlers from one location to another. I have like twenty-five settlements or more (missing three, I think) so theoretically I might be able to send all the settlers from one place to another. I rarely have more than 11 - 16 settlers at any given time in any given settlement but if I could send twenty settlements' worth of citizens to ONE settlement, that could easily be over 200 settlers.
I suspect, though, that if you try to send a settler to a location that is already at the natural max (modified Charisma + 10) that the option to send will be grayed-out. But I've not tried it. Even if you could, though, you'd be hard pressed to keep them in food, water, and beds and defense and stay under the building limit.