I would interpret 'side project' here as a work project that is not your primary and has low stakes for delivery into production timelines or expectations.
I don't really understand this. Work is usually prioritized. If it has low stakes for delivery into production then it should have a lower priority than other activities, but that doesn't make it a side project.
If you don't let developers prioritize some time to play around with new concepts and ideas and to learn how they work, they'll play around to learn in your products.
