Man, cg and other 3rd party API using tools have the ability to track almost everything in this game from what events people participated in, what the guild rosters were like, what the historic results were...
The response they gave you signals proof of your claim (that not being in a guild does indeed make you unable to access the rewards), and makes it absurd they won't just open up a spreadsheet to see what you should have received and re-send it.
It speaks volumes about how little they are willing to do to help players remediate honest issues they run into with the reward pipeline. I'm sure this kind of stuff makes it really stressful for guild managers and such to know how to maintain their player rosters and stuff.
Kind of goofy and hopefully cg just takes the 5 minutes to find your name on a spreadsheet and manually send you the reva shards at least.