Obviously a recommendation to have a high assertion density does not mean "shotgun-spraying assertions in the code".

If your point is just that the rule could be applied mechanically and without thinking and that would be bad, then that's true, but it applies to everything, not just to the rule about assertions. Someone could apply the "keep functions definitions below 60 lines" rule in a perverse way, by splitting every long function definition at an arbitrary point in the first 60 lines and tail-calling a continuation. It doesn't mean the rule isn't a good one.