My impression is otherwise.
For the rules, none are written down for a very long time. For Isabelle and the Game of Triumphs, my best guess is that Giovanni Cossa could have taught her when he presented her with Marcello's gifts.
Well, "could have" is OK, but the word "could" as opposed to "would" implies other possibilities, such as already having learned it from her husband. That is all I wanted to say. As to whether Marcello is implying that she does not know the game already, your old translation, which I was remembering, is clearly unclear. Comparing the old and the new,, I see that your new translation is rather different and deserves my closer attention. Since not everyone has your new book, perhaps I should quote it:
While I was most pleasantly and courteously discussing with him [Cossa --added later: that should have been Caraffa] of the best and happiest condition of the most serene king, your consort and my only and very observant lord, by chance, certain cards of this game which is called "triumphs" had been offered and given to me as a present.
I do not quite understand what he is saying: the "had been offered" confuses me, when it is preceded by "while I was discussing". Were the cards offered to him while he was in the act of talking with Cossa [added later:
should be Caraffa]? If so, the verb form should be "were offered". Or is he somehow saying that he was discussing the condition of her husband and
that certain cards had been offered to him, with the "and that" implied?
Either way, I cannot see how he is presuming here that triumphs is a game she does not know. Putting quotation marks around "triumphs" might give that impression, but these quotation marks are lacking in the Latin original. I continue
When Scipio saw them [the cards], being a thoughtful and diligent man, he said earnestly that your majesty would be very much pleased by them; and he urged exceedingly and immediately that they should be sent to you at the first opportunity. Thus indeed he affirmed that, after you have given your attention to religion, and to the royal duties and concerns (by which great things, such as yours, are usually managed), you will have some free time. By means of these pastimes [ludi], you might restore and revive in some measure your mind, wearied by many and different thoughts. On account of this fact, in his [Cossa's - added later: should be "Caraffa's] opinion, nothing more pleasant or agreeable could be brought to you.
The grammar here is fairly clear, except what it is by which great things such as hers are managed. I assume that he means religion and not "royal duties and concerns".
What is he presuming here, about her acquaintance with triumphs? It is possible that he is presuming that she does not know this particular ludo.
But one might from the same language ("By means of these pastimes [ludi]"), infer that he is not presuming she knows anything of card games. (What is the antecedent of "these"?). It seems to me that he is trying not presume anything about what she does in her free time. He presumes only that she spends much time with her religious and royal duties. He doesn't want to presume more than he needs to. Even if Cossa [added later
: should be Caraffa] told him she liked to play cards, or triumphs, he, a stranger, can't presume to know what Isabelle does with her ladies in her free time. He has made it rather difficult to read between the lines, in my opinion. But of course I am quoting your words (translating Marcello), so perhaps you can tell me what you think they mean, or presume, or imply.