That's the general sentiment of this thread -- that it's not universally accepted.
I'd refer to the Oxford English dictionary, but your citation from the Merriam-Webster contains what I understand is the defining feature:
> "a word ..."
With three examples of words (ie. pronounceable strings).
The addenda of 'also: ... other things' is a bit of a cop-out. This is the distinction between a dictionary that strives to provide definition of language, versus recording popular (mis)uses.
NATO, radar, and laser aren't brands. NATO is an organization, like FBI. Radar and laser are technologies. IBM and BMW, these are brands.
Though I'm not sure that should make a difference for the definitions I'd say the following:
* An acronym is an abbreviation.
* Not every abbreviation is an acronym.
Example:
* FBI is an acronym and an abbreviation.
* Feds is an abbreviation, but not an acronym.
https://www.merriam-webster.com/dictionary/acronym:
"a word (such as NATO, radar, or laser) formed from the initial letter or letters of each of the successive parts or major parts of a compound term
also : an abbreviation (such as FBI) formed from initial letters"
https://en.wikipedia.org/wiki/Acronym:
"The broader sense of acronym inclusive of terms pronounced as the individual letters (such as "TNT"), is sometimes proscribed, but is the term's original and more common meaning"
https://www.dictionary.com/browse/acronym:
"1. a word formed from the initial letters or groups of letters of words in a set phrase or series of words and pronounced as a separate word, as Wac from Women's Army Corps, OPEC from Organization of Petroleum Exporting Countries, or loran from long-range navigation.
2. a set of initials representing a name, organization, or the like, with each letter pronounced separately; an initialism."