The 16" nickel is debatable. There are a lot of factors that can alter those numbers and I find it unlikely.
Once you dig a hole in the sand and cover it up it is no longer packed. Some beaches like up in my area have a lot of black sand. River bason here is all black sand and ib machines will get about 8-10" depth max.

What I do know is as good as smf's are on the beach the pi's can still go deeper but at the cost of lack of discrimination.

Casual beach hunting an smf or even single frequency machines can do the job. If your hunting solely on the beach then either a pi or excall will do better than a general purpose machine.