The comparison is still valid even if the units don't quite match. For example, if you flip the comparison you get "The US's total gross domestic product for 20

The comparison is still valid even if the units don't quite match. For example, if you flip the comparison you get "The US's total gross domestic product for 2018 is equal to 20 times the Apple's total market cap" which is a pretty reasonable statement. If you want to talk about Apple's net income per year, that is a totally different statistic.