Fortunately, thanks to the magic of DVR, I was able to scroll back and see what the poll was. It was the latest conducted by Monmouth, and the results showed bad news for Trump --
Should Trump be re-elected?
Do you approve of the job Trump is doing?
It turns out that all the "Trump's poll numbers were holding" meant was not that they were holding strong, but rather than the House impeachment hearings hadn't affected poll results much, and that the poor numbers were holding to what they were before.
Several hours later, MSNBC had a different, but more accurate -- though still a bit misleading -- chyron across the screen. It said, "Impeachment probe hasn't moved the needle on 2020." That's true. But it bears making clear that the needle had been very low.