No. Snap is not a proprietary format at all. It's a squashfs file with a small amount of metadata. You can make a snap by plopping a file in a folder, add a simple snap.yaml which describes the application and then "make" a snap with the common mksquashfs tool.

There is room in the world for flatpak and snap to co-exist. We created snaps as an evolution on from clicks on the Ubuntu phone, and it covers use cases that flatpak wasn't designed for.