Some spray-on pest repellents will produce this effect. But it is temporary and will eventually fade with time and rainfall.

I am not aware of any arborvitae pathogens that will produce a similar effect so I doubt you need to worry about any disease issues.

Your landscaper needs to come clean with the product that was used. If the product is labled for use on arborvitae and no mention made of discoloration on that label, then you have reason to call the manufacturer.

If the product is not supposed to be used on conifers in general or arborvitae specifically, then the landscaper didn't read the label.

Another scenario is that the product was not mixed properly.

Anyway, a full disclosure needs to be forthcoming so that you can study the label and decide what happened. In any event, I would absolutely call the 800 number on the label, describe the issue (or send picture), and find out if the damage is permanent or temporary.

That marking is most definitely the result of some sort of spray application. Any sort of cultural or pathogenic issue would never present in that very precise line of demarcation. Nature is far too random for that sort of precision:-))