They probably did but only the bone survived time

Likely durability. A bone and a stick can both be thrown into a bag and carried with you, but a bone is much more durable than a stick. It’ll be less likely to break or wear down as it rubs against everything else in your bag.

Likely durability and portability. Think of it as something they use month over month and just mark the day with something like a string band. Bone would be light enough to keep with you, strong enough to not break, and common enough to be available for household use.

That's exactly what is meant, but they have to find something to complain about

Sure, you can say "man" means "mankind", but when you use gendered language like that, most people picture a couple of caveMEN sitting around a fire carving bones rather than caveHUMANS (edited -- I think it would benefit us to picture all genders around this hypothetical fire). Even though we try to use gendered language in a neutral way, listeners will often perceive the language in a gendered way.