Yes it tracks RBD. It’s more sophisticated and you’re right, the value isn’t there anymore. LHR starts at $150 which is too much vs what BA charges for fixed price instant upgrades. Perhaps this explains why the C cabins seem to be getting smaller.
There’s also a new fixed price option that will show up around 36-48h which is around $180. Probably there will be a “promotion” a few days before advertising 15% off 🥱.
FYI the presence of the fixed price is an indicator there and seats and your bid will clear. If you don’t see the fixed, likely the cabin is full or the flight is oversold.