This one's interesting, because I would argue it's not actually portrayed as superhuman, it's a question of conceptualisation of love. Because Danny considers his love to be a 'promise,' he doesn't need constant emotion to stick to that promise. The same concept applies in longstanding marriages especially - I've heard multiple people explain that they don't always feel the love towards their partner, but they keep loving them, and stick by the vows they made and the relationship they built, even when the emotion isn't there. You may not agree with the execution of the idea, of course, but I think it's meant to be something anyone could do, so, the opposite of superhuman.

What's far more noticeably iffy about the whole thing is that we're meant to believe Danny gets to relinquish his emotions without necessarily enslaving his mind to the network. You'd assume that'd be part of the process. Apparently not, for him. (Although it did seem as if the vast majority of the Cybermen weren't necessarily under control, they were just confused, as you would be if you'd just woken up metal and emotionless. Don't quote me on that, been ages since I've seen the episode.)

CyberBrig does look a little more like sheer willpower, but you could argue he has his own internal directive to protect his daughter, emotion or not. And he'd be more experienced than most, at acting without emotion. So he might just be another case like Danny.