Sorry, I give Gallup zero credibility. Gallup always gives rats a +5 to +10 advantage in any and every poll.
Did you know that Gallup predicted that Hillary would win in 2016, according to its poll?
(PEW) http://www.people-press.org/2016/09/13/2-party-affiliation-among-voters-1992-2016/
There are plenty of others reporting the same thing but I won’t bother with Sabato’s “Crystal Ball” or Wikipedia as sources. They have the imbalance in registration even more lopsided than Pew and Gallup.
I don’t see any reason to doubt the statement. Though its not dispositive, after Hillary winning more votes in 2016 and tonight’s results as reported so far I think it’s accurate. The imbalance is probably driven in large part by California and New York.