Open-source means a lot of things, in particular this product is probably not freely available/distributable (as in beer & speech). Just because they're going to give you some source files that you could read to self host doesn't mean that the product is open source.

It runs on any server that has PHP and MySql. Even your most basic shared server will work.

It's delivered as a zip.

And the source is technically 'open' meaning you can modify the source code when you purchase it, but it's not free, and access to the source requires buying a license, and even then you're limited to the terms of the license. The biggest term that's different from most open source software is that you can't redistribute it.

Just because you can host and possibly read the code, it doesn't mean that it has to be open-source. [1]

[1]: https://opensource.org/osd