Yea, that guide was written before dual skills became a thing. So SSR assault then is assault (L) now, and SSR defender then is defender (L) now. Illuyanka gun is actually defender (M), so you still don't quite get the same hp as an old pure SSR defender. And rush (S).... well, the nice way to describe erush (S) is that it's a cute bonus thing, but it's not something that can hold up on its own like assault (S) or defender (S). Depending on what you read, it's hypothesized to be as low as +1% or as high as +3% to double atk at lv 20. Rush is weird and hard to nail down. And the observation that double attack is rolled before triple attack hasn't even been brought up yet!
At any rate, considering how... lackluster rush (S) is, Illuyanka gun is predominantly just defender (M). So skill-wise, that's like an SR defender in old terms. And then stat-wise and cost-wise it's an SSR. Hence, glorified and more expensive SR defender.
It's kind of hilarious how bad rush (S) is compared to most other skills at (S) tier. I think that I'm starting to get why DMM players mock Wrath bow's FLB (it's the only union event weapon to pick up Rush with its FLB).