Ok, so relevant rules: (sorry I don't have the page numbers I am currently having to use my digital edition
Moving Walkers
Walkers use the movement rules for infantry.Tank Traps
Tank traps are impassable terrain to non-Skimmer vehicles, dangerous terrain to bikes, and open ground for other units.Walkers
Walkers are an unusal type of vehicleOk lots of rules, how do they interact.Unit Types
So far, we've discussed the rules as they pertain to infantry
First up, lets take the rules for Tank Traps
So, they are treated as impassible terrain to all non-skimmer vehicles. A walker is a "Vehicle, Walker", and therefore is a vehicle so must treat the tank trap as impassible terrain.
Ah ha! But, they move as infantry, note that they move as infantry does. Not that they "count-as" infantry, so we look at the rules for infantry movement and they are the basic rules. There is nothing in the basic rules or the infantry rules that enable infantry to move through impassable terrain. It talks how they are affected by dangerous terrain and that they may not pass through impassible terrain.
This enables to take into account the rules for infantry movement and the rules for the tank traps without causing conflict. The walker moves as infantry but is still prevented from moving places by impassible terrain.
Note: when in 40k the term "count-as" is used frequently and this means that that unit or model takes on all the characteristic of another entirely. So, if you take a space marine captain on bike, then you can count bikes as troops, rather than being fast. But this is not what we have here, we have the phrase "walkers use the movement rules for infantry", but it does not count-as infantry. There are several instances in the rule book where it uses "using the rules" these are all telling the player how to do things and rather than re-iterate the rules it just tells you that it does it following the given rules.