I am no expert, but from my reading it seems like 903.11 is a rule for play groups that ARE allowing cards to be brought in from outside of the game considering the leading phrase "if a player is allowed". It is also possible that 903.11 was written first or was written before some amendment to 10 (if one occurred)
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My understanding with commander is that players don't have sideboards, so they are unable to bring any cards from outside the game into the game. 903.11 may be referring specifically to companions, which for some weird reason, are legal in commander.
In casual commander, you can of course do what you want as long as the play group is okay with it.
(Also, I don't play a lot of commander, so my rules knowledge for it may be incorrect.)
Yeah I think this it - it's to accommodate companions; technically the ability is not on a card in your actual deck/battlefield (making Rule 10 not applicable?) but an extra deck mechanic like Un-set contraptions, stickers etc. (although I do not know how "assemble a contraption" works on the one legal contraption card?).
EDIT: Also the inclusion of "traditional card(s)" probably gives companion wriggle room.
Rule 10 might also not apply since the companion mechanic is a special action. Still, it's a bit confusing because "Companion" is itself a keyword ability (702.139), so my guess is that while the companion ability allows you to take a special action to put the card into your hand, it itself doesn't put the card in your hand, so it doesn't apply. Companions are weird.
@xgranade imo this is more of a rule 0 matter. If I had a card like that in my deck, I would get the table's permission before starting and bring a replacement card in case someone objects.
That's fair, yeah. I ran into that a little while ago only having read 903.11 and not realizing the contradiction; people got a bit irate with me, even though it was an honest mistake. Ah, well.