Your evidence is that there are more billionaires when arguing there is upward mobility? We are talking about completely different things. Many of those people started out as multimillionaires and then became billionaires - almost none of them started out poor. And you're focusing on a tiny percent of the population and extrapolating that to everyone as though it's representative of anything meaningful.
The last bit is sus though. "Only inherited after making a million". Sounds like it would include born rich, high education, basic necessities covered, can work on startups without worrying about earning a salary, many such cases.
There can be a lot of upwards mobility (say 10%), and there can be a lot of statics (say 90%)
The datapoint I remembered is that a huge fraction of billionaires wer3 first generation billionaires.
But that may not mean much for the greater part of the population.
So we may be saying things that sound contradictory, but are congruent.