He explains here: https://discuss.grapheneos.org/d/562-google-camera-app-stockos-vs-grapheneos/52. Basically, it's a thing about telephoto lenses needing more light, and both apps handle zooming the same way under the hood, so there actually is no difference between this app and GCam or on Stock.
Here's the quote:
Google Camera's 4x zoom button is highly misleading because the telephoto lens is used at a dynamic range based on light level. Indoors with decent lighting, you usually need to go to at least 4.7x zoom before it will switch to the telephoto lens. It tends to only switch immediately at 4x in daytime outside. The telephoto camera needs more light and it avoids switching to it as early in bad lightning conditions. You think it's not working with GrapheneOS Camera because you aren't zooming far enough while with Google Camera you don't realize that it hasn't switched to it because you're using the 4x zoom button. This is a common misconception. It works the same way in the stock OS and GrapheneOS with both GrapheneOS Camera and Google Camera though. The dynamic switch is part of the underlying OS camera HAL.