I think the explanation of stereotype needs a bit of explanation to make sense.
In early printing, a plate would be assembled from individual letters of type before it was used for printing. However, it quickly became apparent that certain words or phrases were used a lot, so typesetters would create "a solid plate of type" for those words or phrases to speed up the typesetting process.
Nah, I think it refers to the fact that popular printing items, like the Bible or popular books that stayed in print more or less indefinitely would be stereotyped, that is an imprint of the pages would be taken, then a used as a mold to cast a plate that could be used while the type slugs could be re-used for something else.
A large part of the printing business was to print fill-in-the-blank business stationary, like orders, invoices, etc.
So a stereotype came to mean a standard form that you could adapt to whatever context by just changing a few details.
In early printing, a plate would be assembled from individual letters of type before it was used for printing. However, it quickly became apparent that certain words or phrases were used a lot, so typesetters would create "a solid plate of type" for those words or phrases to speed up the typesetting process.