If I asked a random person not in graphics what “a selection” is, they might say something like “the act of choosing a single thing from among many”, not “the group of things to which I want to do something”. If I asked them what “a group” is, they might say something like “multiple specific things”, which is pretty much what a group is in Houdini.
It doesn't necessarily has to be a single thing from among many to be a selection. Choosing three apples out of ten is still a selection.
Groups don't have to be created from selections, though. Groups are quite often procedurally generated with SOPs instead of being selected in the viewport, in which case, they were never selections.
Yes they are a selection. Why is something you manually select a selection and something the software automatically selects in the background a group?…it's still a selection, no matter who or what selected it.
A selection is the result of selecting something, a group is the result of grouping something.