I think in general it may be hard to reach some MAGA supporters because I would assume some of the less **** ones don't use much technology. Somebody's racist grampa probably isn't on facebook. Mpst of my older conservative small-town relatives aren't on social media. I think this may be a problem in political polling in general. Younger more liberal types I assume are more connected, more often have a smartphone on them.
Most of the young Trumpsters who stormed the capital were young men. I assume these are the types that patrol 4chan.
You're right, but the sample that the did get already showed that the average supporter was white, Christian, and over 65. So adding those people in would just re-affirm the results or skew it even more that way, meaning it's under-estimating how old, white, rural, and Christian they are.