Not sure that an UpgradeMe bid offer means seats are available.
It's used as a reserve system in case seats are available. If full, there is no harm from VA offering the bid. However a flight may be booked a month out, then 4 pax cancel/change flights the same day or day before. VA then have the list of people willing to bid at that moment, rather than doing a last minute scramble or letting seats go empty. So it makes sense to take bids regardless of availability as they will only be used if needed either way. Certainly the technology used allows for this.
Best to actually check the website for availability.