Technically a bowl is a "part" of a pipe. Eventually, bowl became synonymous with "pipe." I guess the terminology was ported over to bongs.

I notice bong...vessels, tend to have two shapes - one "bowl" shaped like a little cereal bowl, and one which is conical.

Hence, calling it a bowl or a cone, probably both make sense.

Me, I call mine "Steve" after talk show host and secular humanist Steve Allen.