I’ve seen some awful wheels on brand new bikes, especially cheaper ones. Spoke tensions all over the place. It’ll look ok but will cause constant problems.
The wtb i25/vitus build you’ve mentioned isn’t an expensive wheel build.
I’m guessing the shop have discovered your wheels is bit rubbish. Yes it’s only one spoke to you but they will be wanting to do a proper job but can’t as the tensions are mismatched so they are wanting to start again with it
You could just put a spoke in and hope for the best but I’d put money on constant spoke breakages.
If they did this you’d most likely be back next week with another snapped spoke expecting them to fix it.
I very much doubt they are trying to con you.