The terminology that the actual game rules use is quite deliberate - any feature that gives you access to spells that aren't normally on your list generally contains the text (or text similar): "these spells count as [class] spells for you". This preserves the other general rules abut only casting spells that are on your class' spell list - the perks that grant them add those spells to your class spell list specifically for you.
Book of ancient secrets does do it a slightly differently, in that it specifically stipulates that you can cast the spells suing that feature, and also stipulates how (only as ritual). It does give you blanket ability to copy ANY ritual spell into your book and cast it as a ritual, as well as allowing you to cast any other warlock spells that have the ritual tag as rituals as well. In terms of spell versatility, it's actually a really strong pact.
To grysqrl above, in case that was directed to me - I'm aware that the 5e rules are more restricted (and for good reason), I was just noting that the current implementation in BG3 is not, unfortunately, just a bug - it was a deliberate choice. However, with the amount of feedback people have given on the matter, it stands a good chance of being changed.
