NVDA is a free screen reader that’s pretty good, actually. The blind people I know tell me that it’s quite respectable, and given the price, it’s ideal for web developers to test that things are accessible.
I tried using NVDA with that example and it doesn’t read out any of the tooltips unless I move my mouse over the cells, which is not something I could get away with asking vision impaired users to do. Simply moving around the cells with the arrow keys on my keyboard should read back the text in the focused cell, but it does not.