It's not an internet myth in any way. They actually did do this(yes, they've gone after fanfiction before. They're that damn overprotective). They still will damn well protect their trademark too, like they always have.
You're making it sound like they don't heavily protect their trademark in these kind of situations. They eased up on stuff like fanfiction because they were forced to via parody clauses. The law had to stop them, because it still still unauthorized use of their IP at the time. They're the reason why companies don't need to worry as much. Nintendo's pretty much the over-worrisome one. They damn will sue the faker for unauthorized use of their IP in a far more major way than a one-shot emulation console(which is still, despite what should be a big deal technically, wasn't treated as one). This is more impersonation(and yes, it definitely counts as that) and false advertisement, far bigger crimes regardless. They have way more reason to sue and win in this situation.
The only way they aren't going to sue is that this is real. It's not even a contest at this point on how big of a deal this situation is for them. They won't risk losing their trademarks either. They'll sue the pants off of those trying to advertise a product that is treated as their item without permission as well. If you actually think they'd risk losing their trademarks, you have no idea what Nintendo is like at all. No way in hell that's happening.
Still not as simple as you're making it whatsoever. Because a one-time thing can't even damage them enough due to a single sale either. If the person was making a constant amount, it would actually be damaging. Apples to oranges. It's in no way comparable to the romsite factor, despite this(as they were making significantly more money off of their IP than Soulja Boy ever was in any way). You'd have a point if he made a constant amount of them, but he didn't. It was literally a garage sale level they could ignore because it actually did nothing to damage them at that point. If it kept getting sales, they'd worry more about it, but it didn't. It wasn't continuing to going on, which changes the situation entirely. There's a serious difference in context here. They didn't go after him because it didn't amount to anything actually damaging. It's not rocket science.
It's literally just selling your old console and a bunch of games. It's like if somebody re-sold their SNES Classic at this point. With it happening once, there's little reason to investigate. It's more constant emulation use that can be damaging, because that's when they start losing a lot of money. Despite what they say about emulators, they don't shut them down, just the rom downloading. We haven't had a single emulator shut down either. Going after the roms is what they've been doing lately, yet not emulators itself. It was the games itself that Soulja Boy really was profiting from that mattered... and such a tiny drop in the hat and not constantly making and selling these(which would've been a big deal) they don't risk losing any trademarks or anything, nor have to protect their IP in any way because it's not actually a big deal to them. They're picking and choosing their battles. Otherwise they'd have to go after garage sales, which this is an actual equivalent to(if the situation changes, sure, but it hasn't).
Yes, what he did was illegal. That's undeniable. But that's not enough reason to go after him alone. It's a case of picking and choosing what to go after, as they aren't risking anything at this time over a garage-level sale. In fact, garage sales are not proper re-sells compared to business, so they could theoretically go after people doing the same thing. They don't because it's not worth it. Not all things are. This one is not even worth their time(unless, again, the situation changes). Going after the Google ads is definitely worth their time because it's on a grand scale of false advertisement and impersonating companies. They have a real reason to care here. They already learned going after small things isn't worth it alone, as they had a lot of issues with the Youtube situation(which is part of why they overall eased up). The only reason this faker isn't going to get sued is if there isn't a faker. It's a pretty damn high-level crime overall(and one they'll take more seriously than Soulja Boy's dumb idea, which is at least somewhat easy to ignore without problems at this time).