No, it's not open source, which is why the repo for spark is private. It's a paid for commercial app and allows you to use it on a single website, or multiple websites, but the keys and registration are tied to your github account.
He probably means you should use packages that already exist to replace spark functionality. I personally haven't used spark myself so I can't really help you there. However, what kind of app are you going to develop? It seems to me that Spark saves most of your time with billing and such, are you planning to use spark mainly for that?
Otherwise I can recommend you take a look at some of spatie's packages, they are very diverse in usage and allow you to create your own perfect set of packages. https://github.com/spatie