It’s not, if you make the changes correctly.
The point of the issue here is in my opinion that every time you install a new Garuda spin on an existing EFI system partition, the installer (actually the GRUB installation step) creates a new Garuda folder there, which overwrites the existing one. So the old bootloader is lost.
You change both the existing folder name and the GRUB_DISTRIBUTOR and the new installation won’t cause troubles.
Or if you already have two systems but one bootloader (the one of the last installation), you make the change for that second one, chroot into the first installation and reinstall the GRUB, possibly, but not necessarily, following that procedure again after, to use also in this case a different name (or you’ll have a normal Garuda name).
Of course it’s a critical step and a somewhat complex process; that’s why multibooting is discouraged and not supported here. We can’t spend much time solving user-created problems for something we suggest not to do.
But I’ve done it more than once with no issues, and for sure i’m not the only one.
PS:
You may want to forgive me. I thought I was responding to a similar topic opened that same day here !!! 
If your systems are bootable you don’t need to chroot. Just login and apply the procedure in both. The concept applies anyway, in my opinion.