Assumption was that if the OP's reservation was a package reservation (which in non-covid times might include dining), adding a name to the package reservation would trigger the need to pay for the complete package for the extra name.
As you point out, there are no dining plans being offered in these covid times (and I assume Disney will resume selling the dining plan at some point). It was a reasonable assumption (I thought of that reason, too, initially for the OP's reluctance) but the list of what normally is offered in regular business and what's not in these covid times is not widely considered common knowledge.
But since the OP mentioned a package and dining's not an option currently, it might be a room/ticket combo and every name on the package must have the same components for all the days on the reservation. Thus it's reasonable to assume he doesn't want to be forced to add the cost of tickets for the vendor's name to get on the reservation.
So FtW Mike made a reasonable assumption (dining/tickets?) because not everyone has every nuance memorized as to what's being offered and what's not in these covid times. Obviously it would not have been a problem if the reservation at the Fort was room only.
Finally for the OP, that's a good point you bring up and one that hasn't come up here since WDW reopened. You might try to call and ask somewhere at Disney (or try to get a written response to an email). In the non-covid times, a guest with a reservation could meet a trailer or golf cart vendor and ride in with the vendor in their vehicle when they drop out/set up.
But it does present a conundrum.
Bama Ed