He wasn’t undefining the subscription, but the tripwire Subject. To my eye, that’s unnecessary (esp in this toy example) but good style, because if the Subject emits unexpectedly, it could have consequences for everything still listening to it. If you undefine it as part of your teardown logic,you guarantee yourself that a source of hard-to-debug errors will not exist.
All that said, I’d already seen the article @rapropos linked, and I wasn’t convinced by it. I think there’s a benefit to forcing yourself to know exactly what you’re tearing down in your teardown logic. But it’s a very clever idea. Maybe if you have 50 Observables on your page, and you need to edit some in and out in each update? But I only have a few Observables per “code piece.”: (Though I have a couple dozen providers in one project, so I just segmented things that way.)