They don't know anything special. They are just trying to get hesitant high info Republicans to come out for Trump. Besides, all Thomas has to do is wait until 2019 and he is statistically ensured of an R senate to block anyone too liberal (and possibly block literally anyone until 2021).
What's the guarantee that the Republicans will win the 2020 presidential election?
To protect from a Hillary Court, the Republicans would have to retain the Senate for as long as the Democrats are in the White House and simply leave the vacant Supreme Court seats vacant.