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.
Fixed for 10.4! Lots of our older wrapper spells aren't very precise about what you can target with them, since afterward the script interprets the command and actually triggers the spell correctly (and sometimes we still want to allow you to target someone out of range, so that we can then let you know that you're out of range or display a range circle or similar). The real culprit here was that Guardian never cared whether you cast something tagged as a buff or helpful effect—it triggered when you targeted an ally with anything, including totally untagged "spells" that have no gameplay effect, like wrapper spells. So, Guardian now correctly only triggers on helpful ally-targeted effects, similar to Summon: Aery.