I applied for the airdrop, but I'm confused about one point: if it is ERC20-based, then how does it use a tangle instead of a blockchain? Doesn't ERC20 imply that it is based on Ethereum's blockchain, and not a tangle?

ICO Alert: How does the Oyster token (PRL) function within the platform and why is it needed?
Oyster: The browsers of website visitors hunt for embedded Oyster Pearls, PRL tokens, across the IOTA tangle. The private seed key of the Ethereum address holding the PRL is embedded in an uploaded file’s encrypted data structure. As the nodes search for PRL via proof of work they inadvertently re-attach the data to the tangle which extends the file’s retention. This means that the data retention is fully decentralized and motivated by the economic incentive of a continuous blockchain treasure-hunt. The Oyster smart contract only allows PRL to be gradually released from the token’s embedded state, therefore prolonging the treasure-hunt and guaranteeing the data’s integrity for the correct amount of time. This mechanism guarantees that if a user pays for data to be stored for two years, it will persist on the tangle for a minimum of two years.