Okay, so it's as I expected and even applicable to my initial reply.
If it's related to Bitcoin's Merkle tree, it would've called exactly "
merkle tree"
But the message is just "
Merkle" with an uppercase starting character indicating a name instead of a term.
The service is simply taking advantage of a common term used in some cryptocurrency just like what Coinbase, Blockchain, etc. have been doing.
For the actual function, it's just a private mempool, analogous to a Bitcoin mining node that doesn't broadcast transactions that it accepted privately to other nodes.
But Bitcoin and Ethereum works differently so don't take it exactly like that.