Sorry for asking stupid questions. But, does it mean, it does not work with Bitcoin Core node and requires an Electrum node only?
you can set it up to work with anything you want if i am not mistaken but the problem is that bitcoin core nodes run by other people don't let you use their JSON-RPC and using an Electrum node is a lot easier as they are designed to this type of things.
you can also always use bitcoin core's regtest mode though. it won't require downloading anything since it is local and you can generate as many blocks as you want within seconds. https://bitcoin.org/en/developer-examples#regtest-mode