That's certainly how it used to be; bronze was able to board at the same time as gold, silver and business. The new system holds bronze customers back, although it's far from consistently applied.
I guess this is a situation where the bronze "perk" is worth comparatively little, with a likely high percentage of gold and silver cardholders and a sizeable proportion of passengers travelling with cabin baggage only (whether on a HBO ticket or not) putting a lot of pressure on the limited overhead locker space.
Same thing at LHR last night - general boarding was happening and there had been no call for Bronze EC. Wondering if BA are planning to stop Bronze using the priority boarding queue at all and this is their way of drip feeding the bad news?!