I got the number from this page: https://ballotpedia.org/Election_results,_2024:_Analysis_of_voter_turnout_in_the_2024_general_election
which lists it as "eligible voters," which is in turn based on data from here: https://election.lab.ufl.edu/2024-general-election-turnout/
which discusses the methodology:
The denominator for these VEP turnout rates is constructed by estimating the voting-age population or VAP (everyone age 18 and older residing in the United States). Ineligible populations are subtracted from the VAP consisting of non-citizens and felons (depending on state disenfranchisement laws). Eligible overseas voters are added to the national VEP estimates only as no reliable method exists to apportion these eligible voters to states.
So no, it's not just registered voters. It's their best estimate of how many people are old enough and not disqualified.
I mean, Laura Loomer has literally called for 65 million people to be fed to alligators, so this would practically qualify as a moderate position by current GOP standards.