No, but it does mean that you don't know about deferred script execution and possibly do not have a lot of experience in optimising page performance.
Perhaps, but as with any question like this, it's a foolish interviewer who draws such a broad conclusion from such a narrow data point. For example, as it happens I do know what <script defer> does, but that includes the fact that it can't be used reliably before IE10, which is why I've never actually used it on any production site. Not having encountered that particular functionality would therefore have made absolutely no difference to any project I have ever worked on, nor had any relevance to any senior people I was hiring to work on them with me.
Or you had other ways of indicating scripts were meant for processing after the document had been parsed (which might be smart or even essential depending on your support targets).
That might have been something that senior devs only did 5-10 years ago, rather than something senior devs do now, though if that's the case one might wonder exactly what we mean when we say "senior."
But it didn't seem too obscure to me. Deferring execution is a known technique, so I would have probably guessed it right or switched it up with async.
No, but it does mean that you don't know about deferred script execution and possibly do not have a lot of experience in optimising page performance.
Hardly the end of the world, but if you're looking for a senior developer you might really want that skillset.