Kaggle was a great recruiting play; given that Google bought them, and not a recruiting company, I would bet that this is an acquihire.
If they were covering costs at 15M+, that's not too bad and a pretty serious business. You'd hope for a multiple on that (and the investors would be wanting some level of return too). So for met that puts me a the bottom of my range (~6x multiple).
You're right though, they may have just hit a ceiling and the multiple could be a lot lower. I was taking a bit of a punt on what I saw. You've also got to consider the cash component, earn-outs, etc. If you put all that together the 6x or more doesn't seem off-base. It's also the kind of setup that might have multiple people interested in acquiring them.
Another more positive read on "ceiling" is that is they wanted to execute on a bigger vision and the Google acquisition gave them the platform to do so.