I think the right interpretation would be
roger proved with haki alone you could become the strongest . luffy obv doesn't have a comparable level of haki yet.
and kaido is saying with abilities alone , That can't be done.
the right counterargument to his statement was if luffy became the strongest without haki . He is clearly using haki with his g5 attacks.
to the big mom point , she got beat in a team fight ....not entirely sure those are fair circumstances .