Thing is though, if this was true, cordless phones would have caused the same problem. Many of them are the same frequency as wifi.
I think he did say cordless phones were a problem.
Personally, I don't have wifi switched on most of the time or have DECT phones in the house. I use my mobile on handsfree or speakerphone.
Sure, there's no concrete evidence they harm you, but absence of evidence is not evidence of absence.