He was using <b> instead of <div> because it was shorter.
That's a 4 byte savings on each button. If you had a lot of buttons on a page...that could end up saving hundreds of bytes.
I'm not sure how much that matters in the long run, but it's a great attitude to have. Bytes matter, microseconds count.
I started this comment purely tongue-in-cheek, but convinced myself of the argument before I finished writing it.
Not sure why he was trying use <b> rather than a <div> or why he stuck it on the bottom instead of hanging it from the top, but the latter approach intuitively seems like it would work cross-browser.