AFAIK, intro skip markers is 100% enabled by the server owner. If you invite another user to see your content, they cannot force your server to perform intro skip. This theoretical other user can access available intro skip markers if your server made them, but shouldn’t be able to force it to run.
Credit skip, though, I can see a possible point. In the future, Plex plans to allow you to upload intro/outro skip markers to the cloud (Plex themselves) so your server won’t have to perform the operation again for the same file. And in theory, if someone else has the exact same file (same signature), then they immediately “download” the skip markers from Plex so they don’t have to perform it themselves. This could possibly be that your server did calculations for someone else, but again, it was YOUR choice to generate the intro/outro markers for your content.
