You're correct that it went without saying in that people did not actually say it.
Way to not cite Solar Designer though.
Are you referring to the key expansion property that Scott mentioned downthread? He cited the appropriate source: the HMAC specification. The properties he's talking about are obvious: the key is zero-padded to the block size and, if longer than the block size, hashed.
I'm curious if you can find any references to the key expansion / hashing collision before Solar Designer started talking about it a little while ago.
If they're obvious you should be able to find dozens of references. Because they're obvious.
Still interested if you can find an earlier cite for HMAC collision. Most of what I find implies mad crypto cleverness, not this silly little construction.