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That seems a little over-stated.Even the Gutenberg press does not exist! This is a medieval, feudal society without bookbinding technology.
I would venture to guess that it means "without Renaissance-era bookbinding."
Even Bronze Age cultures had primitive books - sometimes made from bark-based cloth, as with the Mayans.
While book tech is primitive (and non-unified), there are such things as thieves' tools and locksmiths, herbalists, alchemists, wainwrights, and gongfarmers (Table 1-3, page 15). (Presumably the gongfarmers sell the muck from privies to tanners and alchemists.) And there are grave-diggers, who will have a lot of work cut out for them, what with all the 0-level characters returning on their shields, not with them.
So all this looks like a fairly specialized economy, with a considerable level of engineering and urbanization.
All skill checks seem to refer to Table 1-3, if I am interpreting page 52 aright: "If your character’s background supports his knowing such a skill, you may attempt a skill check."
It would probably be difficult for individual judges to amend Table 1-3 if the group decides that "tanner" is a separate occupation and needs to be treated as such. Possibly occupations such as "Trapper" and "Weaver" might be interpreted to include tanners. In other words, if the group wants a new profession, possibly the judge can just look at the closest fit in Table 1-3.
Thus the catapult-maker might be classed as a blacksmith, the professional builder might be classed as a ditch digger (because well-planned buildings require foundations, drainage, etc.). Alternatively, the judge might rule that player-characters must always come from humble skill-sets, and must never have catapult-building skill or the like.