The site is more like on open source contribution on bitcoin community. I check the site and it just depends on sponsors as everybody pointed out. It's also good to note that they are offering API which is very helpful to other companies.
https://github.com/mempool
It's a really awesome thing that despite how bad some people paint the Internet, we can never ignore the fact that there are people that would still ignite the spirit of volunteerism, it's one thing to keep a website up and pay with your own money but to do such complex stuff and do it besides your other works that are also taxing to your body and mind is another thing, that's why when I can afford to do it, I always do some donations because I know how hard it is to do this kind of stuff you know. If I recall correctly, aren't they open-source
They are, check their github repo, there's an easy installation of their site there which will tell that its fully open source project.https://github.com/mempool
