https://nocdn.peakd.com/@andablackwidow/the-hbd-limits-explained-technical

When hard limit is reached, the internal price of HIVE becomes artificially high, meaning that on chain conversion and other calculations will cause code to give out less HIVE than one would expect (so f.e. the user trying to convert HBD to HIVE on chain will be at a loss compared to the market, but the chain as a whole will be safe).

A protection mechanism like this exists.