How can you be so surprised? As you said in your preview, "the field, frankly, is not that strong."
This seems exactly what we should have expected from Assefa. She was a really good, but not great 800 runner. She ran (barely) under 2 a few times, but didn't make it out of heats at PDX in 2016 or Rio. Yes, she won a DL race, but it was a "promotional" race (ie, not a "real" DL race for DL points).
For comparison, her PR is a full second slower than Phoebe Wright, another really good but not great 800 runner.
She decided to move up to the roads, and spent the next five years re-tooling. In that time she ran 5 races, again with good but not great results. She started racing for real this year. She ran better than in 2019, but still wasn't among the best -- at Riyadh, in her marathon debut, she was a well-beaten 7th, 8 minutes behind in a not top field.
Then yesterday, on a fast day on a fast course, she ran a fast marathon, beating another good, but not great field.
What have I missed?