Search is awesome. I agree, gmail's is great. Apple's Spotlight is also fabulous. So why does the article say "search doesn't fix this?"
The problem is when search extends across domains. I can search files with Spotlight, but not my google emails. On my phone, it's worse because there are so many different comms systems. I can search my Whatsapp messages; my SMS messages; my email; Facebook messenger; Skype.... Each individual search is fine, but no one tool does them all. Hence, the idea of using manual labels to classify stuff - including emails - can still be useful. Crazy that we're forced back to this.