Most if not all commercial Blu-Ray discs are infected with DRM (Digital Rights Management) designed to prevent you copying the disc, most countries make it illegal to copy such discs even for the purposes of backing it up even if you have paid for a legal copy.
Blu-Ray is far worse both in the quantity of DRM it has and the requirements it imposes. While even ordinary DVD discs also have their own type of DRM at least they don't also impose requirements on the hardware such that technically speaking it could be considered to be illegal to playback a genuine paid for Blu-Ray disc in a Mac, let alone copy it. Old timers may remember Steve Jobs describing Blu-Ray as a 'bag of hurt'.
To playback a Blu-Ray or DVD disc which has DRM aka. copy protection one needs to 'decrypt' the contents. This is fairly straight forward for DVD discs so much so that the protection is effectively useless being that so many tools exist to copy such discs. Blu-Ray has tougher protection and certainly tougher legal terms. In order for MacBluRayPlayer to actually play back a genuine paid for disc it is actually effectively breaking that protection, it does this partially locally in software and partially by looking up already obtained codes to break the protection.
Note: It is not copying the disc but still has to be able to decrypt it to play it.
So as they said and I confirmed it has to do this partially over the Internet - hence you cannot play discs when offline.
What they are talking about is even more illegal as far as the movie studios and the law are concerned which is to completely decrypt the Blu-Ray disc and save its decrypted contents on your hard disc - much like many people (illegally) do for DVD discs. There are some tools to do this but discussing how to do this here is against the rules due to the general illegality of such approaches.
(God knows what new nasty tricks the movie studios will get up to when Blu-Ray UHD starts shipping.)