No, the maps offered here are in exactly the same format as the Garmin "official" maps and all other third party maps based on data from OSM (including the Garmin's own TopoActive and related maps that use OSM data).
This is no different than asking "if I install a third party App on my iPhone or Android, or a third party application on my Windows PC, Mac, Linux or Chromebook, will it invalidate my warranty?" The answer is of course not!
Don't believe me, here's a quote from DC Rainmaker:
"Note that while Garmin would prefer you buy the maps from them, they actually go out of their way to allow you to download maps from 3rd party providers. They simply note that it’s unsupported (but doesn’t void any warranties or such). Meaning that if the 3rd party map data isn’t perfect, it’s not their fault. But here’s the dirty secret: It’s the same underlying map data."
If I was offering a firmware download for Garmin devices, or if the maps changed the firmware, then the answer would be yes, but I'm not offering firmware and nor are the maps modifying the firmware in the Garmin, just like Garmin's own maps do not.
So, the answer is still no, it has no effect on your warranty for your Garmin device.
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