They absolutely do hold non-Prime packages. But as the GP noted, I dropped Prime last year and have not seen any impact on free delivery times. They were holding back packages for me, even as a Prime member, causing 95% or more delivery times to jump from the two day estimate to the reality of over a week.
They gave up trying to provide 2-day shipping to our rural address over two years earlier during the pandemic, and they never restored that supposedly core feature of Prime. Given that behavior, I now see them for what they are: typical rent-seeking crooks.