Soft attack should be inefective on hard targets.
But hard attack should do full damage on soft targets.
There is no reason why a tank should not sloughter 200 men with rifels.
The combat mechanic is that a division has a softness stated in a percentage. Each firing phase it is by chance selected whether the target presetns is hard of soft point. So if a divison has 40% softness, there will be a 40% chance it will present its soft point each firing phase (one firing phase takes 1 hour). Each firing phase a divison has x number of soft shots or x number of hard shots. so if a division only has 5 soft attack it can only fire 5 shots against a soft point. the target then has a defensive value where each defensive point has a chance to avoid the shot. If there are more shots than defensive values the chance to avoid being hit is much smaller (but there still is a chance).
A divisions softness values is determined by the units in the division. A light tank is not 100% hard.
The above is how HOI3 combat works . which Podcat has stated is, with small changes, the same in HOI4.
So having a pure infantry division against a tank division where the infantry has high soft attack and low hard attack it can only fire a few shots when the hard point of the tank division presents it self.