You are very confident with your capital F facts yet your citations are biased and are sloppy with the numbers.
By all means, feel free to provide better citations and more accurate numbers and reports if you feel there is some issue with the ACLU's reporting.
Does this mean that 100,000 documented possession incidents occurred, for both blacks and whites and that the police let off the white guys in higher frequency?
No, and nobody is suggesting that it does, it's a comparison of arrests specifically. Per 100,000 is the standard way to represent per-capita statistics. It's how data is normalized in any remotely competent report. Quick napkin math: 716 out of 100,000 means there were roughly 336,520 marijuana arrests vs a black population of about 47 million. This is a very large sample size and it's easy to see how disproportionate black vs white arrests are for marijuana offenses.
When the numbers skew so heavily towards black arrests, knowing how many white vs black people were caught with drugs and not arrested is not going to change the conclusion in a meaningful way - though you can find reports on these topics too (spoiler: white people are more likely to get off with warnings or probation vs prison time for the same crimes).
Further, the report is unfocused and can't seem to make up its mind whether it is making an issue of drug criminalization or racism.
It's both, these issues are intertwined. The overarching issue is how racism (in policing, in prosecution, etc) drives increased incarceration for a marginalized group, which contributes to further marginalization of said group.
Your graph on incarceration rates is laughable and reminds me of this comic
It's telling that you object to the presentation style of the graph rather than the data it represents. This was a random google image search. Here is another graph:
https://www.apa.org/monitor/2014/10/incarceration Again, the US has a much higher rate of incarceration than any other developed western nation. Only Russia comes close and we're significantly higher still. Even if you disagree that race plays a prominent role, and even if you disagree that drug criminalization is a problem, what is the justification for having incarceration rates that are 4.77x higher than the UK?
(PS: I use the UK as an example because they are culturally very similar to us as a nation, as is Canada who have even lower rates than the UK.)
https://en.wikipedia.org/wiki/List_of_countries_by_incarceration_rate - most lists I find seem to report the US numbers fairly consistently. Russia's numbers seem to be all over the board, as of a
2020 report they were almost as high as the US, but appear to be about half on this list. As per Wiki's list, we're on par with (a bit higher than) countries like El Salvador and Rwanda.