The validity of the underlined ‘but’

The validity of the underlined ‘but’

The passage below is from The Library A Fragile History by Andrew Pettegree.

Although there were few who could match Niccoli’s generous lending, it was expected that humanist scholars would open up their libraries to others. This was one of the crucial tenets of book collecting that the scholars passed on to their patrons, the political and ecclesiastical elite. The humanists could point to many examples from classical Rome that indicated the lengths to which Roman generals and emperors had gone to amass great public collections. That these libraries were circumscribed by a limited notion of openness we have already seen, but to the Italian princelings, who believed their world to be Rome incarnate, the building of impressive libraries was a challenge that they readily accepted.

Conjunction ‘but’ is usually used to put two contrary ideas against each other. But in the last sentence with ‘but’ I don’t see any contrary ideas.

Let me explain my thought.

Of two ideas of the last sentence.

? Roman generals and emperors who built libraries seemed to limited the access of their collections to the public.

? The Italian princelings who were eager to model the Roman ancients were ready to accept to build impressive libraries.

(Of the two ideas, am I right?)

Now my confusion comes about from the fact that these two ideas don’t seem to have any opposing ideas.

Then why is there a BUT in this sentence? What does the purpose this BUT serve?

Thanks in advance.