It likely is. Goku deemed SSJ as the best state and stated they'd work to get rid of its restlessness. Ppl interpret it as Goku drawing, like, SSJG2's powers without needing to buff, like, getting a heightened multiplier without the SSJ suffering drawbacks since SSJg2, as energy consuming as it was, was still useful in battle against = or slightly stronger opponents for sure. I see the logic behind it which is why I just don't dispute whoever sees it as a higher-than-SSJ power up similar to grade 2, but without buffing up and consuming much energy, though I think that, by training as SSJ, Goku didn't necessarily take more of the state in terms of multiplier, but maximized the gains he would have within the state. Like, the progress rate was quicker going down that route, and turning SSJg2 was not possible once the Saiyan has gone down the route of mastering the SSJ.
Though I see no problem with whoever, let's say, has it as a 75x multiplier, maybe a similar (off base state) boost the grade 2 would get since there is logic behind it at least.
Chapter: 391 (DBZ 197), P7.2-7
Tenshinhan: “…Hey, Goku and Gohan were Super Saiyans just now, right? But even so, they were very…how do I say this?...Natural-feeling…”
Piccolo: “…I think there’s no doubt that they were Super Saiyans…However, they’ve trained so that they can exist in that state at an ordinary, everyday level…”
Trunks: “S-so then…When they fight, they’ll [perform] an ev-even more tremendous transformation…!”
Vegeta: “Are you an idiot?...You don’t seem to think things over…They’ve judged that state as the best! If they get used to that as a matter of habit, then even if they raise their battle power, the strain on their body is very small! [ ] They’ve thought this through...!"
I take that statement as Vegeta shutting down the notion of MSSJ being # SSJ... Just that in terms of strain, it differentiates from the SSJ.
Though I've seen people taking Vegeta's "even if they raise their battle power, the strain is very small" as meaning to raise above SSJ's capabilities but still being confined to SSJ's low strain and lack of need to put their bodied into stress, that is, accessing the power he and Trunks had done without undergoing a body change and leaving the strain as a small thing. I feel that is valid too.
But since Goku turned into SSJg2 in the RoSaT but said that all in all, SSJ was the best, without hinting at it being the best in a way it could access higher powers like grade 2 without buffing up, just that it was more balanced, that the overall narrative was just focused on SSJ being still the best state due to a more in tune balance and that training in it could extract its full potential not in terms of multipliers, but power gains. The power up in SSJ they could get from mastering the state despite the multiplier remaining 50x would just make it more worth than getting a 75x power up or so but the SSJ itself not growing as strong as it could. That's my way to rationalize Goku not working on SSJg2.