I'm just indifferent with it. It wasn't a impressive prank but it makes me wonder about that damn sauce. When did Mulan came out again ?

It wasnt, Rick and morty is already a popular and beloved series, and even the AdultSwim stream said "Here it is, so you can stop asking us about it"
It clearly was a prank and it also fits with what was said on the season 2 finale of R&M "Turn in to Rick and Morty season 3 in, like, a year and a half" it almost was a year an a half, off by 3 days.