It's a joke you autist. Let me explain it.
Pooh is "living under the name Mr. Sanders". In one sense, this is literal - he lives under a sign that says "Mr Sanders" - but people in real life would "live under" assumed names that were not their own. This was common-ish back before everyone had a driver's licence in their wallet and needed to submit their fingerprints to every banker and airport security officer that was paid to care about such things. But a child wouldn't understand this, so would only ever be capable of understanding the most literal usage of the term.
There. The joke is ruined. Happy?