Since you cannot leave the Last Name at Birth empty for a WikiTree profile, Erichsson will have to go in as a surname even if this may not be strictly correct from some points of view. It won't get mixed up with the preferred name.
It is trickier when there is both an actual family name to go in the surname fields and a patronymic, like if he was Simon Erichsson Lang.
When these names are imported by GEDCOM they often end up with Erichsson_Lang in the field for Last Name at Birth. This destroys the search functions of WikiTree pretty well, so for this reason it is better to have only Lang as Last Name at Birth. But where to put Erichsson?
Well, in my opinion it is best to put it as a middle name, even if it isn't what we think of as a middle name today. The patronymic is, after all, a name that comes in the middle, between the given name and the family name, when a person has all three.