It is when you don't have any proof of such a conversation taking place or the context and tone of what might have been said. Add to that being unwilling to name the alleged or give any facts about whether they challenged that person's alleged comments at the time and how that person responded.
It ought to have been settled at the time even if that were left as 'beg to differ' or no offense intended. Unnecessary to bring such matter out in public as they've produced no evidence. Even the allegation is airing on the side of the recipient choosing to interpret something as insulting rather than something derogatory being said.
It's a calculated attack by means of being deliberately vague to cause maximum effect across the family. Delivering trial by media where the public only has one sides word to go on.
In this case there's also the accusation that the reason why Archie doesn't have the title "prince" is because he's "black". An accusation made despite knowing the real reason (protocol established in 1917 limiting the title to 2 generations from the monarch except for people in the direct line of succession) and explicitly refering to that reason.