They also have Battle Altars powered by handfuls of powerstones, it's only the oldest Battle Altars that are exclusively fit for Orbs.
The Orb Flex is more about managing something no member of the Colleges ever has than changing the strategic situation on the scale of "turning Battle Altars from irreplaceable to replaceable".
All right, so I'm seeing two lines of arguments that are not necessarily shared by everyone on the pro withholding orbs side:
1: Demand for orbs is satisfied and thus they have no immediate use (see Deathbybunnies)
2: Orbs are redundant with powerstone arrays and thus are of limited use. (see quoted post above.)
So, presuming I understand these arguments correctly, to the rest the thread at large:
Both of these are effectively saying that the colleges won't benefit that much from having some earlier, to which I can only say hogwash. The entire premise of the orbflex is that orbs of sorcery are actually a big deal, and the colleges are distinctly uninterested in applications of magic without clear financial or military applications.
Boney has said that magical feats you can use an orb for can also be done with an array of powerstones - this is rather distinct from saying that they can be replicated in the same conditions, or that they do not come with engineering or performance compromises.
And even
that is distinct from saying that lives will not be saved by activating another 8 orb of sorcery-powered altars. It is implausible to me that there are not some sitting in storage somewhere ready to go when any of them would be artifacts worthy of study, and likewise implausible that taking their main component from irreplaceable to replaceable would not change the calculus of risk for how they are used...
...not that that calculus would prohibit their use
anyways if we accept the idea that they're redundant.
I do not think Mathilde's prestige is worth withholding 8 battlefield-shaping assets from the empire for half a year... nor do I think her prestige would be helped on net by the revelation she did so simply so she could present it in tandem with the book on AV.
Wherever anyone reading this falls on the spectrum of "Asset too valuable to field" and "redundant with powerstone-based equivalents", the orbs are a big deal as a practical matter, not just an academic one. It does not make sense to withhold them.