Yes, that particular example does show specific code that is ONLY ran upon an interrupt signal, thus relatively safe to leave in the main loop. But then this is just an example, not a larger project that may have many such interrupts, etc. and thus coded in various different ways.
The “clean void loop” is a valid and important recommendation, not a hard and fast rule… as experienced programmers CAN get away with some coding methods that will trip up inexperienced users.
I am NOT an experienced programmer… more like an insufferably OK self learner 
But here is an example of needing code in the void loop()… but set in such a way as to ONLY run under specific conditions, and even then ONLY once every prescribed timeframe.