We already know they have a certain number of low quality stones...

We already know they have a certain number of low quality stones as this has been mentioned at previous EGMs. No source is going to give 100% high quality stones and the 19,000 stolen were not even intended to be sold at the upcoming auction (it is likely why they were still on site).

Rather than see this announcement as indicating that they are sitting on mostly low quality stones, it says that they are aiming to only offer the high quality stones at the first auction.