Honestly the main thing to me is that Nintendo is handling the DLC choices, where as there is no indication anyone but Sakurai and his team are handling Spirits. And while it is true that Spirits, being largely just PNGs, could have potentially been added after the DLC was decided, at this point we have no idea at what point in development happened. So any guesses as to whether Spirits could have anything to do with confirmation is speculation at best. If DLC was decided first, then potentially Sakurai could leave out characters that were chosen in order to keep that image available for the Fighter Spirit. However, if Spirits were decided first, it seems highly unlikely that Nintendo (or at least, whoever from Nintendo is making these choices which is another unknown quantity) would really care about characters already being represented as spirits. Furthermore, the only pre-2018 first character that makes sense that we haven't seen (as far as I can recall) is Elma. And if she wasn't in the game but other Xenoblade X spirits which we have seen are, that would be a dead giveaway she's DLC. Which would be very un-Sakurai like to not hide stuff like that. So really, if Spirits do deconfirm the character, that would mean that the characters would have to either be unreleased games (or at least recently released titles) or third party characters.
So essentially, I feel like it's way too early to tell if this is a rule. It's impossible enough to predict Sakurai, let alone an unknown number of people giving Sakurai a list and him deciding if he can make them a character. I would just not worry about it too much.