My guess is that the server just assumed that the two people dining were the two sharing the room. I don’t know whether you making the reservation would make any difference as that bit would have been dealt with by the person who welcomed you rather than the server.
They may have made an error, or they may have just arranged things in a way that helped you. These things can happen and sometimes they do charge things to the wrong accounts, but it can’t be relied upon.
More often than not, in my experience, they ask for everyone’s card when they do the bills, and often everyone gets their own to sign, so this sort of thing wouldn’t accidentally happen in those cases.
The key thing is that they didn’t charge two credits from the same meal to one account. Many of the sharing questions on this board are where people re asking if four people can turn up and use both dining credits for two of those people. I’m not even sure whether the system would allow this to happen even if they tried to.
Whilst the credits are not transferable, it is certainly possible for things like you experienced to happen, and people could arrange things to try to get them to happen that way, but it is certainly not guaranteed.