It’s not open-source in the way that you can’t read its solidity code, because they didn’t upload it.
You can just see its compiled EVM instructions:
Ethereum Account 0xf97e0a5b616dffc913e72455fde9ea8bbe946a2b Info
The Ethereum BlockChain Explorer, API and Analytics Platform
But you can of course reverse-engineer the EVM code. It’s just a matter of time until someone does. In the end not publishing its source won’t matter.