I agree with your central point that HBD supply and virtual supply must be kept in mind here, but HBD in the DAO does not count towards the debt limit. HBD is only 5.3% of the Hive market cap (by the way measured on chain for this purpose) when the DAO is factored out.

If the supply of HBD keeps rising rapidly without a corresponding increase in Hive price, we could see it reach that limit fairly soon.

In theory the contraining supply should force the price of Hive to rise, but it is still quite possible that it does not, or the price is not forced up enough compared to increased demand (and thus supply) for HBD.