Why would they need installer IDs? The question is if they collect it, not if they need it, and all their other behavior suggests that they do collect it.
They can probably reach the same conclusions about why there might be more installs than downloads by thinking about it for maybe 5 seconds instead of tracking people.
Easiest explanation off the top of my head, without reading the article, would be IT departments including Firefox in their base image they use on all their standard issue computers, resulting in hundreds and possibly thousands of different installs having the same download ID. That alone by itself would cause an absolutely massive discrepancy between download and install numbers. My company includes Firefox in our base image and it's on at least 200,000 different laptops and desktops, with a handful of different download IDs between them depending on when they got issued the computer.
