Why would we mistake him, though, if we don't know about the whole balrog slayer bit? Is there some other thing in common the two have, and only the two of them? We may as well mistake Glorfindel for any other super-Elf we know.
Also, is there a canonical source about Ecthelion's appearance where anything more is said than the one passage about his silver armor and shining helm? Glorfindel's hair is definitely said to be golden somewhere or other, but Ecthelion's appearance is more ambiguous, yet he is depicted with dark hair everywhere in art. Presumably due to that being a shared trait of he and his kin. If we have anything definite on this, we can rule him out in-character definitely.