They didn’t dismiss the option entirely. In fact, the statement from that guy Miller was an open threat without any nuance. He basically said: “We have no plans to start a third party now, but that will depend on what elected Republicans do in office.”
P.S. — It’s clear to me that Trump sees a third party as Plan B. Plan A is to take over the GOP directly.
Which is just the way it should be. You leave the threat hanging especially before the impeachment vote and whatnot, without actually doing it.
Even Mitch quickly turned round and voted to dismiss the sham impeachment when Rand Paul brought up the vote.