I agree with this. It's hard to gauge what is acceptable because it really depends on the application. So many other dependencies when dealing with latency and how it effects performance.
Unfortunately, you HAVE TO do it. If you do not set a threshold for what is acceptable, how do you determine whether or not your are providing an acceptable experience to your users?
No amount of aggregate metrics can help you answer this question unless you know whats acceptable, and what isnt - for each important set of URLs in your app.
I agree that its "hard" to do. In our own product (https://www.leansentry.com), we solve this problem by grouping urls, and using good defaults / making it easy to override the thresholds for users.
The post is about selecting a top level metric for monitoring website performance. One a problem is indicated, you would definitely need to drill in to figure out what part of your app is affected, when, and what caused it.
LeanSentry (our own application monitoring product, https://www.leansentry.com) does this. However, describing this was outside the scope of my post (but you can see the demo of it on the website).