I too believe that the percentage of high volume users is higher. The graph shows the ammount of users that access the internet regardless of the device. I assume that about 2/3 of those users access the internet with a smartphone exclusively, implying that these users don't use an ad blocker at all, since the word hasn't spread to an extend that everyone uses one.

The remaining third of all users access the internet with a browser on a desktop computer. It is very hard to estimate who uses ad blockers, as I can't find any reliable resources. It depends on the peer group one is a part of. When I look into my social group, only three out of ten users do have an adblocker installed (me included) - and for two of those I had them introduced to ad blockers and set it up for them.

On the other hand: I assume that the combination of Firefox and UBlock Origin would apply to 90% of Fediverse / Lemmy users.