Is it true the the windshield wipers are pawred by the air in the spare tire?

No.

The washer fluid reservoir receives air prssure from the spare tire.

I had a member of the MO-KAN VW Club just tell me a story of this exact thing... I believe in the 50's they did!

The windshield washer fluid tank sat behind the spare tire which was located in a cavity under the front hood. When you pressed the dash button for the windshield washer fluid, air pressure in the spare tire would power the windshield washer fluid. A quite ingenious setup. See item 4 in the attached photo.

Here's a further explanation from the 1970 Volkswagen owner's manual. See items 3 & 4.

I worked on dozens of Bugs in the 60's and 70's....and it IS true.