I wonder how many of those around him got the reference… Despite its popularity, Undertale references in real life seem to fly above most people's heads still:

To be fair, the Delta Rune looks like a clear homage to Zelda anyway.

In this case I can see the confusion from someone who's at least casually familiar with Zelda. Now if we were talking like "someone things the face of Sans is Jack Skellington" then I'd be going "I take it you've never been a Hot Topic in the past 16 years have you?" (they ALWAYS have Jack Skellington faces there)