I think they're missing the point... Are they worried about being attacked one-sidedly from underground? Or about being attacked in the rear with a spiked mace? He may have done both before, they're not actually related in any way. These sheltered sect disciples are really...
This is hilarious. It's also nice that it shows that Changshou isn't all-knowing and infallible. He can occasionally make oversights or be caught off guard, even if it's just over how dumb people can actually be.
To be fair, the chance of a spiked mace up the bottom would give me second thoughts as well.......
Overgrown_Forrest