In 2018 AppStore revenue was $46.6 billion [1]. Apple revenue for 2018 was $265.6 billion [2]
So, your "rounding error" is in reality 17.5% of Apple's entire revenue. It's "only" twice bigger than the entire Mac portion of Apple's business which is $25 billion [3]
$14b (app store napkin estimate), out of $265b (2018), is ~5%.