I don't believe that's true at all. I think the core demographic are people who think Space Marines look totally radical and tubular to the max. Kids still say tubular and radical to the max, right? I think the majority of people who get into the game first select the army they find aesthetically pleasing. I picked the Imperial Fists, Necrons, and Imperial Knights because I think they look sick as hell.
Given the increased number of people who are into Warhammer either through computer games, fiction, or the table top miniatures games over the last two decades I'd say their strategy is working. GW has also worked to include more women and people of color by including them in the fiction, the artwork, and even as examples of miniatures in their ranges. (Though there are valid complaints that this has been done to slowly.) They've also toned down some of the decidedly non-kid friendly aspects of the setting by getting rid of (ignoring) previously published lore and ceasing production of certain models (Juan Diaz daemonettes being the best example).
Getting people to buy new models is part of it. But the Primaris Marines also represent something we hadn't seen in 40k for a long, long while. New technology. Well, recovered technology I guess. Not just with the Primaris Marines themselves but with some of their equipment and vehicles.
Okay, they have a broader market now than they did 20 years ago. I don't know how broad the market can get, but we'll see.
Marines look cool to people who like ur-fascist iconography combined with space warriors. That's not a moral failing or something, it's aesthetics, but that's the point. Generally people who are minorities (of any kind) are going to be less into it, particularly because the kind of people who attack and oppress them often tend to be into that kind of iconography. It's notable that peak space-fascism was pre-Primaris though - Primaris definitely go space warrior a bit more and look, well, more American and less "Crusader in Space".
That said, as oppression relents somewhat, that iconography becomes less prevalent and less linked to hate. But let's not pretend white supremacist iconography and the Imperium of Man, especially the Marines and factions of the IG, don't have an enormous crossover. That's the whole reason we're having this discussion.
Interestingly re: racism I feel like GW was very definitely anti-racist in the early 1990s, for example, they basically made out that the Deathwing were mostly people from a Native American ethnicity/culture (admittedly this is crude 1990s anti-racism but still), but sort of gave up on it the 3rd edition to like 6th edition period, and even retcon'd that example. I'm sure they're anti-racist now but I think they're a bit confused about the whole thing.
As for the broader market, most of that has simply come from not being such stingy idiots with their IP. They used to have an utterly paranoid attitude that if any game company made basically a good game with their IP, it was a threat to them, particularly if it was a wargame. Hence them not allowing TW:Warhammer until AoS took over on fantasy. But they've clearly realized that was wrong, and are now actually working with Creative Assembly to leverage the new Oldhammer stuff. Their IP still needs to broaden out, and pushing Primaris Marines and the Empire isn't how that's going to happen, frankly. Not unless they do the long-suggested plotline of the Emperor regaining consciousness and going "WHAT THE ACTUAL F...." at what the Imperium, or the whole Rebel Marine thing. I do think one those may well eventually happen.
As for "the first new/recovered tech!", well, not really. All through Rogue Trader and 2E that was happening all the time. It just got shoved down with the general attitudes of the 3rd edition era and onwards.