I don't think that's accurate, just google up an Apple ][. But even if it were true, it wouldn't be strong evidence for your 'consumer hardware platforms become more open' theory given the reality of the longer-lasting smartphone era we live in now. Both the theory and the example used to support it seem starkly at odds with that reality, to me.
Game devices and smartphones were carefully designed to remain closed. Although the Pinephone and Librem 5 look like smartphones, and use many of the same components, they are far closer to phone-sized PCs. And so they're open from the start.
They were more open than an iPhone, but they were not particularly open to anyone with less than a CE/EE level of computer and electronics expertise. They certainly were not designed to be modular and open like the clone PCs that came later.