Very interesting discussion. Others here note a general question whether FOI laws are really intended to address such documents. But I have also another angle:

Generally a public preservation grant will have conditions, especially if EU funded (not mentioned but well likely as EU finances huge digitalisation efforts).

I'm surprised the author does not mention these grant documents which financed the digitalisation. I would expect these to have provisions on sharing/making available/publication. If they do you have a separate case: breach of contract (of either national or EU provisions). This would lead to financial penalties and generally wake the institutions up.

https://www.europeana.eu/en

https://digital-strategy.ec.europa.eu/en/library/basic-princ...