I read some of the article you linked to. Which is to say, I read enough to know the article is attempting to justify by a speaker manufacturer it's policy to not cover certain damaged drivers under warranty.
They are not even really wrong to not cover the warranty, but the explanation is weak, and should not be relied upon.
You would be well advised to take fmw's advice and learn to ignore power ratings in watts as specified by loudspeakers. There is no standard or agreed method to measure or specify this value. In some cases the power rating of a speaker is a number out of thin air, or perhaps the marketing department's desired number that they feel will sell the most speakers.
Speakers handle power either by creating sound pressure levels, or by creating heat. The SPL is desired, the heat is undesired. Period. Full Stop.
Exactly how much of each, and what ratio it happens at, not only varies from speaker to speaker, but varies based on the kind of program material the speaker is being asked to play, and also varies as the speaker is played (heat accumulates, which alters the ratio such that the speaker plays less loud and creates more heat over time).
Once the ability to dissipate the heat is equal to the heat being generated, than even a single watt more means the speaker is failing to some degree because that heat is attacking the physical materials the speaker is made of, and heat failure is permanent.
How many watts is that? Well, it's not a fixed number, but if you could measure it at that point, that would be your power rating. For that day. With that song. Using that amp.