"Wailing on them" isn't enough of a similarity to be considered related attacks.
Well, really, how in-depth of a similarity do you desire?
Again, the fundamental thing they have in common, is that they're both basically "canned animation" attacks. They both make it so that upon connecting, the opposition can't do anything to escape the attack, forcing them deal with it in its entirety.
On that basic level, yes, they have more in common with one another, then they do not. Overtly similar concept, different means to an end. Parlance for most FGs goes that, the only thing that really separates the two is that one registers as a "(unblockable) throw" [SGS], the other as a "(blockable) hit" [Ranbu].
Besides, what version of Shun Goku Satsu scores only 7 hits?
If you're simply looking for a Shun Goku Satsu "type" of attack? Hanzo's "Weapon Flipping Technique" in Samurai Shodown 5 Special (where he basically punches you quickly, breaks your arm using Koppo and then Izuna Otoshi for the finish) does only like 5-6 hits, at most.
But in any case...how does that factor into the equation, exactly?
I mean, the definitive first example of a "Ranbu", the Sakazaki-style Ryuuko Ranbu from Art of Fighting 1, did easily around the 10-20+ hit range (no hit counter existed in that game, so an approximation was used on my part). And of course, variants have appeared in the years and games to follow, which have usually only added more hits, or "worse" (especially if you're a SNK boss).
Meanwhile, the Shun Goku Satsu, and the various similar attacks it has spawned (like Morrigan's Darkness Illusion, and the WFT attacks in the Samurai Shodown series) have ranged any where from like 6-30+ hits, depending on the version of the attack and the game it's in.
Some fighting games have even gone forth to incorporate both "schools of thought" into one game, further blurring the thin line that separates the two. Capcom's own "Jojo's Bizarre Adventure" fighter is an excellent example of this type of thing in action, and it certainly is not the only one to do as such.
So, again...what does the fact that Omega's version only does 7 hits have to do with anything, really? Omega's attack is an undeniable, direct homage to what could be considered one basic concept, that's been in fighting games for many years before Z3 and ZX were released. Omega's version just does it in its own "way", suitable for the game's means. It's both unblockable like Akuma's Shun Goku Satsu (and has its trailing shadows on start-up), yet it also has a lil "hop" upon activation like Ryo's Ryuuko Ranbu. Omega's version freely invokes both, as part of its presentation.
So, far as most in my ilk are concerned, the SGS and Ranbu are, as suggested above, little more than slightly different approaches to the same/similar end. Y'know, kinda like how projectile attacks, whether they be executed like "Hadoukens" (thrown in the air in front of you), "Power Waves" (thrust along the ground) or through some other type of means, are both considered the same thing, just executed in differing styles.