Comment by rmah
One of the key components of metal movable type printing presses is an appropriate ink. The concept of using a press to print on paper was well known for 100's of years across the world from China to Europe. Movable type had even been invented earlier, often using wood or ceramics instead of lead. Getting the ink right to work with metal type was not simple and recipes used by various printers were considered trade secrets (though obviously leaked as printing spread widely and rapidly).
Many inventions are like this. They seem simple in hindsight, but at the time, required putting together tools, techniques, materials and insight from multiple sources. There's an old BBC TV show called "Connections" that explores the origins of many modern technology and the often strange paths that led us there. For example, without people loving perfume, internal combustion engines might have taken decades longer to have been developed.
One of the fun things about writing fiction is that I don’t have to stick to the natural flow of events the way history actually unfolded.
Kind of like how some countries in Africa skipped landlines and went straight to mobile phones, I can let the Romans stumble onto just the right ink recipe a bit early.