You don’t do what Google seems to have done - inject diversity artificially into prompts.
You solve this by training the AI on actual, accurate, diverse data for the given prompt. For example, for “american woman” you definitely could find plenty of pictures of American women from all sorts of racial backgrounds, and use that to train the AI. For “german 1943 soldier” the accurate historical images are obviously far less likely to contain racially diverse people in them.
If Google has indeed already done that, and then still had to artificially force racial diversity, then their AI training model is bad and unable to handle that a single input can match to different images, instead of the most prominent or average of its training set.
This is the result of capitalism - corporations (aka the rich selfish assholes running them) will always attempt to do horrible things to earn more money, so long as they can get away with it, and only perhaps pay relatively small fines. The people who did this face no jailtime, face no real consequences - this is what unregulated capitalism brings. Corporations should not have rights or protect the people who run them - the people who run them need to face prison and personal consequences. (edited for spelling and missing word)