I imagine they meant because of finite resources; SE probably wouldn't have the bandwidth to devote teams to two simultaneous massive remake projects at once, so if they ever start on an FF6 remake , it wouldn't be until the FF7 was done, and probably not until it was proven to have ended up profitable for SE (so it might never happen if the FF7 remake somehow doesn't sell enough to make up for the expense).

So they meant they wished SE did FF6 first as a guaranteed remake project instead of FF7. :P