It's a real rule. I looked it up and it seems several credible sources (LATimes, Thesaurus.com, etc) say you go with the sound, not the letter or in the case of "you" not the way the letter is being used. So it would be "an youtube video," although to my ear that sounds incorrect. OTOH, "An hour" is correct, but "hour" doesn't sound like it starts with a vowel sound to me.
This reinforces what you say, but it explains why “a” is used before words that start with the consonant sound of a y:
“The indefinite article ‘a’ is used before words beginning with a consonant sound, including ‘y’ and ‘w’ sounds,” writes Garner’s Modern American Usage. ‘The other form, ‘an,’ is used before words beginning with a vowel sound. Since the sound rather than the letter controls, it’s not unusual to find ‘a’ before a vowel or ‘an’ before a consonant.”
That’s why “a,” not “an,” goes before the “European.” Though “European” starts with the vowel “e,” it’s pronounced as though it starts with the consonant sound of “y.” Similarly, that’s why “an” goes before “MBA.” Yes, “m” is a consonant. But the letter is pronounced “em,” beginning with an “e” sound.
A European.
A yew.
A useless piece of shit.
A Yahoo account.
A yellow bus.
A ukulele.
A willow.
A whim.
Going with the sound OR the rule (the rule that if a word or syllable starts with a consonant sound y, you use “a”), you end up using A for “you” or “you’re”.
This is precisely why saying “An YouTube video” sounds wrong to you: It IS wrong. Remember: “several credible sources (LATimes, Thesaurus.com, etc) say you go with the sound, not the letter or in the case of "you" not the way the letter is being used.”
VERY few words currently in use have a vowel sound y at the beginning of the word. Yttrium is one. “AN yttrium atom” is correct. But no exception has to be followed, as it’s pronounced “itreeum” which is a vowel sound.
As an English major, I’m sure you learned that things sounding “right” will almost always tell you they are right.