Well, broke access to Google Play services anyway
Well, apparently after he first patched / flashed
Well, after he reinstalled OS
The statements I found confusing (actually, I just think they're incorrect) however, were:
GApps don't install to boot... ever... So they wouldn't be in boot whether installed, extracted, stock, patched, backed up or other!
Except that there would be no difference! ... Boot doesn't change at all after flashing GApps. Neither have it.
TWRP backup boot.emmc.win file is an alternative to downloading / extracting boot image from a ROM for those with compatible TWRP, but resulting image should be identical!
Nb. A Magisk zip will alter the partition and subsequent backup, but GApps zip won't! They're two very different beasts.
GApps work fine with stock kernel, ramdisk etc so don't require any changes, and a GApps package is just a means of distributing often stock-standard Google Apps for legal and practical reasons.
Magisk on the other hand requires code injected into a ramdisk to boot with SU privileges, so necessarily alters a boot image, whether boot or recovery.