It would (and does anyway) affect the boot, yes. But not in the way as you seem to think.
It would need to be present and correct to trigger (thus control) the resume process and grub would fall through and do a normal boot sequence and not do the resume. My understanding is that the file must be present and assumed formed right to trigger the resume. If the file is not present or ill formed then a normal boot happens and I assume the BTRFS work around does not remove the file since it is a normal boot and it should not be there in that case.
Your understanding is wrong.
grubenv is not at all needed to trigger the resume.
You need a correct resume= option on the kernel command line to trigger the resume, and that is (should be) part of the menu entry in grub.cfg.
grubenv is only used/necessary to set the correct menu entry to be booted (in this case it’s the default one anyway, so grubenv is redundant regarding this), and to hide the boot menu so that the user cannot select a different entry by mistake (this could even cause data loss).
And the second part is a problem with /boot on btrfs, as grub2 cannot remove that entry from grubenv so the menu stays hidden.
But again, this would not have any influence on the resume itself, and the resume script that is run during/after the resume removes that entry in grubenv as a workaround for btrfs.