They charge him $.15/MB to delivery the book[1]. Cell carriers charge you ~$15/200MB = $.075/MB for data. Certainly a markup, and it gets better as more people use WiFi, but not 129000%.
But remember, they're charging that $0.15/MB even for people who don't own a 3G-capable Kindle. In those cases it really is roughly a 120,000% markup over an already profitable service.
And those cell users can download the book multiple times at no additional cost, in perpetuity. So sometimes they may be taking a loss. It's really not all that simple, and I think calling it a 120,000% markup is dubious at best.
[1] https://kdp.amazon.com/self-publishing/help?topicId=A29FL26O...