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.
Thanks to you and others for explaning this question throughoutly! I had a good time finding out what OP actually meant No offence, it was interesting nevertheless!