Safe to share boot partition?
the tweet doesn't explain the reason but it is unrelated to security. it is not 'safe' to patch a foreign boot.img regarding the functionality of Magisk because of the following reason:
Magisk Manager assumes

cpu architecture
encryption type
encryption state
dm-verity

read from host device is equal target device, it does the patches according to host specifications (where Magisk Manager is currently running, regardless of the origin of boot.img
while this is bad practise, i want to contradict because it will cause issues only in rare corner cases and is (most likely) 'safe' to patch a foreign boot.img