The suggestion that this is somehow related to WiFi is at best wrong, and at worst completely misleading for a non-technical person. The xDSL sync rate is there to be clearly seen.
Ask for the name of the helpdesk supervisor to whom this was escalated. I don't know if they can access info about the historical sync rates for the line, but if they have that data it should clearly show when the issue first appeared. The line may currently be stable, but that will be because the sync rate has been pulled back so far - a faulty line will often be unstable, but not always, so implying that stability = no fault isn't quite correct here.