I don’t know if they reclassified it at some point, but back on those days 3.5 was titled “Windows for Workgroups” and 4.0 was the first to be known simply as “NT”.
Forget what I said, I recalled an old memory from childhood of a 3.5 upgrade box for people running Windows for Workgroups.
NT 4.0 is definitely what popularized that version prior to Windows 2000 and XP. Most people who just say “Windows NT” are thinking about 4.0.