I'd be highly suspicious that this was, in fact, food counterfeiting in any way. I've never heard of a food brand being counterfeited on Amazon (with actual evidence). Food "counterfeiting" is generally a "legitimate" manufacturer themselves falsely representing their ingredients (e.g. diluting their olive oil with canola oil).

Amazon used to be known for keeping boxed foods for much longer than a grocery store -- so if cereal takes 2 years to expire, at the grocery store you're always buying cereal that was made last month, while from Amazon it might have been made a year and a half ago.

If it wasn't the expiration date, then it was probably kept in a very hot environment for an extended time, which also breaks down flavor and texture. If kept on pallets under plastic in the blazing sun for days, it could cook them like an oven, flavor goodbye. (Might not have even been Amazon, but a wholesaler or similar.)

Everything you've said is far more consistent with time/heat than the notion that you were getting "counterfeit" cereal.