The main issue here is that the number isn't relevant: it's per API key, not per user. So unless reddit starts handing out API keys to every user then it doesn't make the slightest difference to third party apps.
It is relevant. Developers have the ability to throttle and/or monetize users based on usage to pass on those costs to power user outliers, while not impacting the majority of their users that fall into that free tier.
The point is "user" as used in this post is not a single reddit account or a single individual. A "user" is an app which supports few or many users. I would say if you measure it the way most people would mean user, in fact 90% or more of users are impacted by this price increase, because while each individual user will fall well below the usage threshold, they will in aggregate quickly exceed it because it's all being attributed to the app they are using. On the free tier you could support maybe 10 average users before you wind up with a bad experience as their requests start to bounce off the limits (it's not that their average usage would hit the limits, but the spikes would quickly result in requests become very delayed)
