Yes, you still have to know kind of how it was named and kind of how it was used. It is just the rest of the details you can forget (or likewise, don't need to remember). It is really useful when you have to use libraries that are kind of similar but not exactly.
Anecdotally, I use Typescript and code completion is exactly how I discovered Javascript's "some" and "every" methods. It involved lots of scrolling because I didn't know it would be named like that, but based on the presence of other list comprehension methods I had a good suspicion that they were there somewhere...I still often mistakenly write LINQ-named "all" and "any", but the online type checking quickly snaps me out of it. Code completion can actually be improved to deal with these cases with more utility, by not only matching on the parts of the member's name, but a system could also match on synonyms to parts of what you typed as well (or document aka's in the interface that play no other role than hooking into code completion).
Anecdotally, I use Typescript and code completion is exactly how I discovered Javascript's "some" and "every" methods. It involved lots of scrolling because I didn't know it would be named like that, but based on the presence of other list comprehension methods I had a good suspicion that they were there somewhere...I still often mistakenly write LINQ-named "all" and "any", but the online type checking quickly snaps me out of it. Code completion can actually be improved to deal with these cases with more utility, by not only matching on the parts of the member's name, but a system could also match on synonyms to parts of what you typed as well (or document aka's in the interface that play no other role than hooking into code completion).