As described in the post, this was a limited quantity promotion that started during DYGYCON on March 13th and ran for less than an hour. Only people who bought at least 50 packs during that window and also had linked WAX wallets got them.
Hopefully that clears it up for you :) This is entirely different than the standard Untamed airdrops based on how many total packs purchased.
Here is the announcement post from March:
https://peakd.com/splinterlands/@splinterlands/ultra-rare-horrors-of-the-splinterlands-promotion
OH damnit!!! I hate when I miss out on stuff like this! Thanks a lot for clearing up my misconception!