"When people talk listen completely" <- isn't there a comma missing there?

I was innocently poking around on wikiquote and I came across this tidbit:

When people talk listen completely. Most people never listen.

Ernest Hemingway as quoted on wikiquote

Which at first I thought was some accidental editing mistake. Like any good wiki user I starting gearing up to fix it, especially when I noticed it used to have a comma in the history but being the paranoid guy I am I thought I'd confirm the quote independently. And I found these:

• http://timetowrite.blogs.com/weblog/2014/01/hemingway-on-how-to-practice-to-be-a-good-writer.html

• https://bibliophilewannabe.wordpress.com/2015/07/22/ernest-hemingway-on-listening-and-obsering/

• https://www.brainpickings.org/2013/11/13/hemingway-on-writing-2/

All have no comma. Am I crazy thinking there should be a comma between talk and listen?

Far be it from me to second guess the great Hemingway but I think this is from an interview. So it could be this isn't his writing I'm questioning. It's his interviewer's.

Anyway, I'd just like to know if this is one of those subjective things that could go either way, and so should be left alone lest I start an edit war, or if this is objectively wrong and should be changed.

In writing, the comma is necessary between “talk” and “listen” because the phrase “When people talk” is a dependent adverbial clause. It depends on the independent clause “listen completely” (consisting of the imperative “listen” and the adverb “completely”), which forms a complete thought, or sentence.

According to Purdue University,

Use commas after introductory a) clauses, b) phrases, or c) words that come before the main clause.

When the snow stops falling, we'll shovel the driveway.

In speech, the speaker would indicate a comma by briefly pausing after saying the word “talk,” rather than immediately following it by the word “listen” (as though the two formed a compound word). By reciting each variation, the necessity of the comma will become obvious.

A comma becomes necessary to include a pause to separate the two clauses.

When people talk, listen completely.

In the above sentence, there doesn't seem to be more than one possible placement of the comma leading to conflicting meanings. As there is no alternative, the sentence can be understood even when the comma is missing.

But one should take care when a sentence could provide more than one meaning depending on where the comma can be placed. There is an urban myth that comes to my mind which contains the following line.

Hang him not, let him go.

The final decision made by the jury was to declare that the accused was indeed innocent. While this statement was spoken, the clerk who typed it misplaced the comma.

Hang him, not let him go.

Although it doesn't seem grammatically sound, it made perfect sense to the executioner to hang him and not to let him go. Hence, a simple comma can make the difference between life and death.

According to this article https://owl.english.purdue.edu/owl/owlprint/607/, one of the usages of comma is to prevent possible confusion or misreading and I think this one needs a comma to prevent such thing and you are right.

The quote's provenance, according to Wikiquote is

Ernest Hemingway, letter of advice to a young writer, reported in Malcolm Cowley, "Mister Papa", LIFE magazine (January 10, 1949), Volume 26, No. 2, p. 90. A longer version appears in Hemingway's Across the River and Into the Trees (1967)....

I haven't been able to locate the magazine online, and as far as I can tell, the quote doesn't appear in ATR&ITT. If Cowley quotes the letter in Life but doesn't tell you where it was published, I'd withhold judgment about Hemingway's authorship. If there is a letter, especially a handwritten letter, then you wouldn't expect the punctuation to be rigorous. If the author left out the comma, those who quote the passage should do likewise.

Punctuation is a matter of style, so there no right or wrong answer. Most style guides will advise that a comma separate an introductory clause, especially if the leaving out the comma induces an impossible parse like "talk listen".

I did a quick search and these three came up with a comma,

When people talk, listen completely. Most people never listen.

http://thinkexist.com/quotation/when_people_talk-listen_completely-most_people/145028.html

http://www.quotationspage.com/quote/2675.html

http://www.brainyquote.com/quotes/quotes/e/ernesthemi383060