In your reference, I think this summarizes the issue nicely:
As others have said previously, the apostrophe is a way to indicate that something in a word is missing. In one case, it may indicate the omission of numbers (ex. '20 instead of 1920). In another case, it indicates the omission of words which may be used to expression possession (ex. 1920’s music instead of "music that was recorded in the decade that began with the year 1920). It is never, never, never used to express plurality.
The New York Times Manual of Style and Usage (1999) agrees with Words into Type about the apostrophe, although about little else:
decades should usually be given in numerals: the 1990’s; the mid-1970’s; the 90’s. But when a decade begins a sentence it must be spelled out. [example omitted]; often that is reason enough to recast the sentence.
NY Times seems pretty reputable and they like the grocers’ apostrophe, your example is some random person’s summary
Could you provide some example style manuals that say that?
I can give you a stack exchange: https://english.stackexchange.com/questions/13631/is-an-apostrophe-with-a-decade-e-g-1920-s-generally-considered-incorrect
In your reference, I think this summarizes the issue nicely:
Too also quote:
NY Times seems pretty reputable and they like the grocers’ apostrophe, your example is some random person’s summary