That's hilarious, but it sounds like those tunes were pitch shifted into the same key. Nashville producers have more skill than that. Oh, and FL/GA line is frigging awful.
The pitch and tempos are shifted, but more nefariously, the pieces of the songs which don't fit the VI-I-V progression are removed and replaced by pieces of other songs which do fit. It's deliberate and meticulous cherry-picking. The only thing all the songs have in common is that they're in 4/4 and the verses and choruses all start on the VI chord. I should make a table showing the actual chord progressions from each song and how the creator deliberately masked out the parts of each song which are different from the baseline the creator chose.
Original mashup: https://www.youtube.com/watch?v=FY8SwIvxj8o