Ok.
“The owner, the employees, and the buying public are all one and the same, and unless an industry can so manage itself as to keep wages high and prices low it destroys itself, for otherwise it limits the number of its customers. One’s own employees ought to be one’s own best customers.”
The gist of which is: instead of relentlessly trying to pay as little as possible to employees companies should try to pay as much as possible to employees while keeping other costs and prices as low as possible.
Ford understood that as much money as possible circulating through the most hands as possible produces a healthy economy.
You correctly point out that increased productivity is what led to Ford's success. But you can't separate Ford's wage policies from his other productivity increasing policies. Not only did Ford double the daily pay he also shortened the work day from 9 hours to 8 hours.
A year later Ford nearly doubled his auto sales. He raised daily wages by another dollar in 1919 and by 1920 was selling about a million autos a year.
So of course he wasn't relying on just his employees buying his cars as you and dismal have been arguing. He was arguing that paying the highest wages possible made cars more affordable for his workers and more affordable for the businesses his employees spent that money on as they had more money too.