I'm going to speculate that some border officers are treating "Software Engineer" as a sham title for "Software Developer", the way one might treat "Waste Disposal Engineer" as a sham title for "Garbageman" (rightly or wrongly).
However if one were to attempt entry as a "Software Engineer" arguing one ought to be classified under NAFTA as an "Engineer" then they'd certainly need a degree in "Engineering".
I don't think border agents are going to allow someone to eat both halves of the cake, i.e., claiming that the term "Engineering" slides an applicant around the limitations of the CSA category, but then claiming that a clearly CSA-related degree fulfills the education requirements for this "Engineering" position.