I'll be nice and limit my direct answer to your question to, "Not much."
To keep it short, I'll just say that they don't understand what they are talking about.
Ekklesia does not mean "the called out ones" anymore than "butterfly" means a fly made out of dairy-product. The entire website is built on what is called the etymological fallacy, which is the idea that we can define a word based on what its parts mean. It is a very common mistake, but that doesn't justify it.
It also isn't true that "church" is a pagan word. If it is, then so is "baptize." Certainly pagan used the words that we get the word "church" from to refer to their temples and services, but that doesn't mean that the English word means that. That's another fallacy that is commonplace today. Have you ever heard a preacher talking about "the power of God" say that the Greek word for "power" is the word we get our word "dynamite" from? To that point, they are correct. The Greek word is
dunamis, and that is the word we get "dynamite" from. But then the preacher concludes that God's power is "dynamite power!" But that's wrong. You don't read the definition of a later word back into an earlier word, which is exactly what these . . . folks (being nice here) . . . are doing with "church."
If they're going to make such basic errors, you would do well to just avoid everything else they have to say. I'm sure there is some truth in there somewhere, but really, if that is their methodology--and worse, if they are actually basing their whole argument on such ignorance--then they aren't worth paying any attention to.
I would strongly recommend to you a book titled
Exegetical Fallacies by D A Carson. It explains some of what I've said above in a good bit of detail.