Primalbase was only on the Waves Platform at the beginning, they added the Ethereum-Gateway later where you can convert your Waves-PBT to an ETH-PBT token. I think this big holder reserves the token if somebody wants to convert from Waves to Ethereum, these coins should not be on the circulation supply additionally to the Waves token. But I haven't checked this ETH wallet, maybe somebody from the team will answer too.