I interpreted it as: with the nature of fans and the associated vibration/movement, some gap is necessary and this is the limit given the precision of injection molding.
Phrased differently: a 0.5mm gap is the minimum possible to also be able to account for the 0.1mm (or whatever) variation in injection molding.
Phrased differently: a 0.5mm gap is the minimum possible to also be able to account for the 0.1mm (or whatever) variation in injection molding.
You're right to question the wording.