Alright this is a pretty old one, and afaik happens to all allied shields spells that use 'spell wrappers', ever since Guardian was introduced. Targeting an ally with Rakan's E2 puts some allied buff on them which allows Guardian to trigger, even if the 'actual spell' never actually started casting on them.
It appears to be decently well known for Rakan mains, so the Rakan in this clip probably used the bug deliberately. +1 for soloQ map awareness!
The fix here would be to stop Guardian from using the engine level targeting thing it uses right now, and have it be managed on script level like literally any other effect does.