That analysis is from 10 years ago. Again, the landscape is different and spending time on this is a borderline foolish discussion (interesting from a technical perspective). It matters if you are the owners of a business, in the mass-retail space. Save yourself the time, add to your features or verticals.
Yes, 5s is still fine.
It seems like your argument isn't that it's fine across the board, but rather that it's tolerable in a few circumstances. If I try and load a website that is packed with features but loads slowly and maybe has a choppy experience, I'm going to go somewhere else if I can. It's only in the few circumstances that you can't go somewhere else will you be relatively fine.
Business to business app, or sales site?
YMMV. 5 seconds could be intolerable, or a big improvement.
If you are running a marketing or sales oriented site, you want to minimize friction.
It sounds like you are doing more of a “captive user” app, similar to the government/healthcare oriented stuff I have been working on the last few years. In that case, you still want to tune things a bit, but you do have goals and criteria which might not match this article.
Make sure things which can be cached are. (And don’t sweat the size of static assets too much, the second visit is just an HTTP header swap)
Minimize the number of requests.
Schedule requests for supplemental data in a component or form to load after the primary data.
Minimize the size of dynamic data messages, though this can be hard when you work with devs who like very long names.
Accept the fact that many of your web clients may have round trip latency well over 200 milliseconds, as well as the occasional multi second delay or request loss, and plan to work around that without hanging.