Whether an asset is "money" or not is subjective. For example, to most people in the US the euro and yen are not money, their money is the US dollar.
My money is bitcoin.
When someone says "bitcoin is not a money", what they really mean is "bitcoin is not my money and it's not the money of most people".
If worded properly, it's a valid empirical observation, and it's why I'm long-term bullish on bitcoin. Adoption is still very low.