Yes Izzy only hosts open source apps, but their policies are more lax than of F-Droid. So they can include proprietary libraries or stuff in forks that the original devs don't want. Also Izzy takes the binaries directly, so they can't guarantee their reproducibility like F-Droid does.
Btw other repos don't need to even be for open source apps. Total Commander for Android has their own F-Droid repo too (although it doesn't work atm).