Yes, Amazon does more retail sales than Walmart. Their "revenue" is only 15% on all third party sales, which are now more than half of total sales on the platform. They don't report on total sales on the platform on purpose to avoid drawing attention to the fact that they are bigger than Walmart.
But that 15% that amazon takes (not counting the extra cut they take for Fulfilled by Amazon items) is larger than Walmart's average markup. So that isn't really an argument against their size.
I'm not confused by it, though I'm a native English speaker and live in an English speaking country and spend most of my internet time on English speaking internet, so maybe that's why