Yeah but you can get naughty times even without her being convinced... in the end she'll run away after one time...
but if you leave the conversation, invade mind, start conversation again than end it and invade mind again only to finally end the process with a last conversation you'll get the happy end... which is detailed in the Walkthrough.
Which is why I pressume those people are able to grasp the concept if they manage to seduce Evelyn for the happy end even before Nell's story reaches that part
I mean, aren't there messages in game that are like: Are you really absolutely sure you want to commit incest with Evelyn because she's been pretty explicit about how deadset against it she is? It's well established beforehand that you'll need to do something to alter her views. Also, the option to try to use invade mind to convince her incest (although it won't work until her dark secret is exposed) is okay appears before confessing your feelings is an option primes the player to know it's an option they can attempt. For Nell, there is no parellel "Leave your entire life behind because I asked you" option available ahead of time to foreshadow needing it (because there's no narrative reason for it to exist).
I can't speak for everyone, but I recently did a full game replay, and I remembered I needed to invade mind on Evelyn to not screw up the relationship, but forgot about it for Nell. I just didn't bother to come and ask because I already had that scene unlocked in the gallery.