Not necessarily. You need not become a member of a political party for a US voter to be called a "Registered [insert party affiliation here]"
Around 60% of US States allow registered voters to indicate a party preference when registering to vote. It is highly unlikely that the party affiliations noted by voters when registering to vote is at that time, or remains, 100% accurate. A registered voter is able to change their indicated party affiliation.
Aside from General & Local Elections, indicated party affiliation (or even non-affiliation) may limit the types of elections for which a voter is eligible and some elections may require party membership.
https://en.wikipedia.org/wiki/Primary_election