The no-fly list exists, with its flaws. The bill (only one of two the sitters-in want voted on) doesn't create new problems and makes no difference to any effort to fix the no-fly list. And as I understand (from watching the sit-in for about 12 hours), it wasn't a Democrat's idea. These two bills were what was on the docket for action by the House. What the sit-in really demands is that they debate it, take the opportunity to review the problems with the no-fly list, and perhaps commit to fixing it. But the problems with the list do not "undercut" the importance of the demand that something be done, something be said publicly, something be put on the record.
And this bill is not the focus. There's another bill, to close the loopholes on background checks. And yes, there are other measures that might make a bigger difference, but they are not yet on the docket for action by our lawmakers. We have to start somewhere.