Amazing that he got away with it for so long, but then for most people trust is the default position.
If you go back just far enough that the internet isn’t always reliable and there’s no (easy) way of checking – plus he’s a nice / charming / helpful guy, it’s not that difficult to get away with it.
The claims are believable enough without being outlandish – things like a Bronze medal in a Junior race, well who on earth remembers things like that?! You’d just take it at face value. It’s not like claiming you won the Tour, everyone would see through that in seconds. Obscure enough that no-one will really go and check it, big enough that it’s still very prestigious.