It may be an error 40, but it still should be easy for steemit website to check if the memo contains an account key (or what likely is one) and give an error message for that.
Why keep an error source open just because YOU would never make that error (which of course say at least 50% of those who make it).
Sure, I'm not saying that we shouldn't try to reduce those chances. Appropriate modification is on its way.