I think he's right. Google's home HTML is only 5kB, which is pretty low these days, and their images and CSS aren't much more. If your page size is less than 10kB, those couple of bytes start making a difference. Google have probably literally optmised everything else they can optimise. Image atlas to reduce it to one file, they have a CDN, Custom web server, custom OS kernel, etc., some of which helps far more than some closing tag.
Yes, all those things are determined by scale, because there is a clear cost involved, that requires a certain scale to pay for itself. However, in the case of closing tags the cost is negligible, so it's something that can be used by everyone. Google claims (however, I still can't find the link that shows) that loading/rendering their search result pages slower makes people perform less searches. That's why they do not allow you to have more than 10 results per page; not even via your personal settings. The same trade off will hold for every service whose profitability is directly dependent on the number of page views. So it's not scale, just the sort of business you are in that determines whether not closing tags makes sense.
