FWIW, I did read to the end of the line, but I parsed that as two separate assertions (with “and that” being a means to introduce the second assertion):

1) it is one of the first games using ECS

2) that you also have an interest in it

Rather than the intersection of:

1) the set of games using ECS

2) the set of games that you have an interest in

Or, expressed another way, it’s this (how I read it):

(it's one of the first games that is using ECS) and that (I also have a deep interest into)

Vs

it's one of the first games that: (is using ECS) and that (I also have a deep interest into)

I suppose I would have written this:

It’s one of the first games using ECS which I have also had an interest in.

I’m fairly certain I’ve almost always seen something like “[of] which” to introduce another qualifier, whereas “and” almost always introduces a new statement that stands on its own (the exception being in more formal contexts where there’s a sentence structured along the lines of “consider such [insert class of objects here] which are both: [this] and [that]”).

I think it’s the “and that” which threw me (and others here who misread you) off.