Dad joke meets programming language pun in its purest form. The old man is questioning if he's a bad programmer for calling C# "C hashtag" instead of its proper name "C Sharp." Clearly, musical notation isn't in his programming curriculum. It's like calling jQuery "dollar sign query" or Python "snake underscore." The audacity of some developers to mispronounce the sacred texts!
Microsoft created C# to sound sophisticated, but they forgot most programmers can barely read sheet music. At least he didn't call it "C pound sign" โ that would've been truly unforgivable.