It’s less about science not being possible, so much as the society of the time not being the kind which would support it.
Look at the actual ancient Greek, Roman and Chinese civilizations. They had a lot of natural philosophers. Some even had plenty of advanced for the time technology; novelty steam-driven toys, for example as well as crossbows in China 2500 years ago. They obviously knew enough to figure out a few natural physics and exploit them with engineering.
But they weren’t scientists. Science isn’t about building a better crossbow. Science is about the systemic methodology of investigation. It’s about a process that requires a well-educated and cooperative elite. The culture of classical era post-apocalyptic remnants that Creation supports isn’t advanced enough to sustain those elites, and in the two places it is (Lookshy and the Realm) the culture there is actively dismissive of investigation beyond a certain point and those who are inquisitive enough to pry past the palatable lies of the Immaculates are either eliminated or (if valuable enough) are inducted into the conspiracy by the Sidereals who don’t have to do Science because they have the collective wisdom of the High First Age stored in vaults they can dribble out to people as bribes.