The 70 million number (the one you were specifically nitpicking) comes from the FBI database, not from a union of state level databases. So that's 70 million people who have been convicted of or arrested on a felony charge, or had a state explicitly ask the FBI to retain a record of their misdemeanor arrest, per your source.

OP was clear in this thread that "criminal record" wasn't synonymous with "has been to prison." This point was also explained to me explicitly during all the pre-employment background checks I have had.