They had a knowledge of the mastodon that was not casual.

Making a projectile point from mastodon cortical bone requires access to mastodon bone in a specific state — dense, healthy cortical bone that has not decomposed or been significantly weathered.

This means they had recently handled a mastodon carcass — either from a previous hunt, from a naturally dead animal, or from a scavenged carcass.

They knew which part of the mastodon skeleton yielded the best dense cortical bone for the purpose.

They had the tools and skills to work that bone into a functional weapon tip.