Could it be, I wondered, an eggcorn? There is such a thing. From Wikipedia, "...an eggcorn is [the] ...substitution of a word or phrase for a word or words that sound similar or identical in the speaker's dialect."
The caveat here is that the new phrase, while different in meaning from the original, must be plausible in the same context, such as "old-timers' disease" for "Alzheimer's disease". If the new phrase does not meet this requirement then it is not an eggcorn. Instead, it is a malapropism, which is the use of an incorrect word in place of a word with a similar sound, resulting in a nonsensical utterance.
It would seem then, that "in prompt to" is a malapropism, and a phrase with no meaning that cannot stand as a substitute for the word impromptu. I have to conclude that my first thought was correct. The writer is a grammatical dolt.
While there can be much humor in malapropisms, or as Mrs.Malaprop once said, "Sure, if I reprehend any thing [sic] in this world it is the use of my oracular tongue, and a nice derangement of epitaphs!", it still saddens me to see the abuse of our beautiful English language accepted as the norm in today's society. This may prove to be disastrous and can lead to our eventual downfall or fall down.
“Ill-fitting grammar are [sic] like ill-fitting shoes. You can get used to it for a bit, but then one day your toes fall off and you can't walk to the bathroom.” ― Jasper Fforde, One of Our Thursdays Is Missing
Take that, and may it serve you well! As the great Winston Churchill once said, “Ending a sentence with a preposition is something up with which I will not put.” That quote has nothing to do with eggcorns or malapropisms. It just amused me greatly.