Swift did indeed coin the word; OED comments Freq. in mod. use, a person lacking cultivation or sensibility, a philistine; a lout, a hooligan. for the interjection, they waffle a bit, saying to compare other int. yoho and yoo-hoo and In some cases supposedly characteristic of cowboys, esp. when executing daring feats on horseback, etc.

but take a look at the very first citation for the int. and tell me that the source doesn't suggest a bit more of a connection to the noun:
1976 Beano 3 Jan. 6/2 Yahoo! I've won first prize in a crossword competition run by a lemonade company!


and this: 1976 in V. Randolph Pissing in Snow 85 The woman riding behind an Indian, who yells ‘Yahoo’ when she grabs the saddle horn to mount and dismount.