> I didn't have time to write a short letter, so I wrote a long one.

First known use in English comes from a 1658 translation of Blaise Pascal in 1657

> Je n’ai fait celle-ci plus longue que parce que je n’ai pas eu le loisir de la faire plus courte.

translated to

> I had not made this longer then the rest, but that I had not the leisure to make it shorter then it is.

(note the archaic then)

This was a popular piece of wit at the time.

Mark Twain wrote something similar a hundred years later

> You'll have to excuse my lengthiness - the reason I dread writing letters is because I am so apt to get to slinging wisdom & forget to let up. Thus much precious time is lost.

But it's still quite different.

There is a great article about this one on quoteinvestigator! https://quoteinvestigator.com/2012/04/28/shorter-letter/

Hmm… this started as an admonition for using then and than interchangeably. I see folks get it wrong unintentionally a lot… then i re-read your parenthetical and pulled up etymonline and had my mind blown a bit.[0]

Seems everything old is new again.

[0] https://www.etymonline.com/word/than