When snow is laying on the ground and you place a block its like flying over the snow

This is vanilla behavior, why would you expect this to be changed?

The mod is about winter unlike vanilla, where snow biomes are one of others and it would be a nice feature to see in mod

I don't really want to do this as it's a very specific change from vanilla It is a change (and inconsistency) from vanilla. This behavior (replacing instead of placing on top) only applies to things that do not collide. Stacked snow layers can be almost a full block and as such it would be quite weird to replace them.

I also don't see the specific reason. Yes this mod is about turning the entire world into winter, but it does not touch snow placement at all. It sounds like a very random thing to try and modify.