Thank you! Happy you enjoyed!
Yeah! Here's a kind of loose explanation. You need to install the "Unity 2D Animation" package. It lets you stick bones in your sprite and animate 'em. You can get that stretching effect on a sprite by adding two bones and animating them to get pulled apart (there are really good tutorials around the web for all the tools in the package). After the bones work, I just pull them apart using code instead of the animator to make the bones respond to the width of the object that's stretchin' the player.