I see don't see where that article suggests that Mars is outside the habitable zone by any reasonable estimate. The only part I see questioning Mars's position

I see don't see where that article suggests that Mars is outside the habitable zone by any reasonable estimate. The only part I see questioning Mars's position suggests that Mars might actually not be as close to the edge as we think. Mars's lack of habitability is partly because of its position, but also partly due to other factors like its lack of a magnetic field and atmosphere. If you somehow "poofed" a copy of Earth into Mars's place, I think it would be considered habitable (but still not really for humans), though I'm not really qualified to say that with any certainty.