I don't think it's that big of a deal. I went through a phase where I thought I might be INFJ because I'm pretty shy and I'm responsible and a planner, in addition to being NF overall. And it does sound nice. It's easy to mistype as something else because it inherently sounds more appealing, as one can't so readily see the downsides of living in the heads of other types. And then there is the old "N > S" bullshit floating around, and whatnot. And it's really not unsurprising that many INFJs would end up here, an analytical, online community of seekers.
I dunno. So someone mistypes. So a lot of people mistype. Each of us are lucky if we've found a type that we feel describes us accurately enough to be comfortable using that label. If they really don't fit the INFJ label, they'll realize soon enough. Until then... eh. All words are imprecise. Language is just metaphor for reality. If they've missed the mark by that much, it will probably come to light fairly quickly.
I guess I am more concerned about why they are seeking use of the label. If for self-description, no problem. If for boosting confidence, it probably would merit a second look. I heard someone once say that your enneatype is often recognizable in part because it makes you feel a bit uncomfortable. I think that's true for MBTI, too. Your "true"(est) type will probably make you feel a bit uncomfortable, because you can relate to its failures and shortcomings.
My INTP dad says something about sports cars like, "if the maker has to identify it as a sports car, it's not a sports car." Like, if you have to hold up a sign and parade it to be recognized as such, something's probably off. I guess I feel that way about type, too. If someone feels compelled to parade around their type and to use it as a shield, there's probably something weird going on that has more to do with psychological unhealth and using type as a self-defense mechanism, rather than a self-exploratory one.
So I assume if someone is psychologically healthy - I don't mean not mentally ill, but rather a more holistic sense of at peace with themself and the world - they will tend to have an easier time typing, because there are few motivations that would cause them to seek out a certain type.