Captain Obvious, "Wanted In Roma" has changed their info since the April 30th articles you linked above. From today's May 1 article:
https://www.wantedinrome.com/news/where-you-need-to-wear-a-mask-in-italy-from-1-may.html
Museums - The wearing of masks is "strongly recommended" when visiting
museums and archaeological sites, according to the websites of some
museums and reports in Italian media. However a statement on the
website of the Italian culture ministry states that surgical
(chiurgica) masks are still required when visiting museums.
That statement, within the info about free Sundays on the Ministry of Culture's site, today reads (translated from Italian):
https://www.beniculturali.it/comunicato/22750
To access monuments, museums, galleries, parks, monumental gardens of
the State, the possession of the reinforced green pass or the basic
one will no longer be required, while the use of surgical masks will
remain. Some locations can be visited by reservation only.
Could that change? Sure, but it apparently hasn't yet at the state museums, monuments, etc. yet. Might private museums have their own rules? Could be but to be safe, best to always have masks with you and be prepared to use them?