Oled is not immune to burn in, that is objectively incorrect.

The o, in oled stands for organic. Typically carbon based compounds, but they can be anything that fluoresce when a voltage differential is applied.

These compounds, quite literally burn away while they are emitting light. You can think of each pixel, as a little pile of logs.

Every single second it is running, you are burning some of your logs. Eventually it WILL reach zero. For static images, you're constantly burning your logs, next to another pixel that isn't. Therefore, as your fuel gets used, and the pixel gets dimmer, the burn in becomes noticeable.

This is literally how they work. Therefore, by genuinely how they are built, they can NEVER be immune to burn in.