Interesting analysis and you make a good point. These are indeed very high numbers for early milestones - usually you see these early milestones much lower and the biobucks allocated to pie-in-the-sky scenarios that assume the drug is going to be approved for multiple indications and then sell big. And as you pointed out, ZGEN retains a large stake to boot. Makes me want to take another look at their drug - I can't say I've ever followed them much.
It's often also the case that the milestone details are not initially disclosed and are redacted in the SEC filings.