-Who's the cat that won't come out, when there's danger all about?
- Shaft!
-You're damn right!

The story:

Look, it clearly says here he's a CAT.

Your average cat, being not a stupid animal, dives for cover if there's danger nearby and doesn't come out till the danger's over, right? That's the essence of cat- like behaviour. Sense danger all about, you don't come out. Good lyric.

So if this Shaft bloke dives for the bomb shelter when he's in trouble and doesn't lift his head till the baddies have packed up and gone, then this is entirely sensible behaviour and not to be derided in any way.