The account is not going to sell STEEM. From the description:
As a reminder, once SBD declines back to $1 or below, the STEEM received by @burnpost (including all post rewards which have so far been powered up) will be powered down and/or burned by sending to @null. None of the coins received by @burnpost will ever be sold or otherwise distributed.
By reducing the overall STEEM supply, this will put upward pressure on the price. Not sure if that really means "None of the coins" or if it should say "None of the STEEM". Unti now, I believe that the account's SBD has been sold for STEEM. It's not really clear to me if he's going to switch from selling SBD to using the "convert" function while SBDs are priced near or below $1.00.
Good point, that language could be improved. Anyway, you are right it won't be selling STEEM.
There is a new update on my main blog describing the procedure that will be followed when SBD is at or below $1.
Well, it will be interesting to see it play out. I am interested to see what he does now.