So the bank's speculative asset loses value: I struggle to see this as money being destroyed as it wasn't actually money, it was an asset with a price attached to it which has now changed. In contrast to money sat in your bank account, the price was never redeemable (you couldn't go spend it on beer) unless you used that asset to get the debtor to pay you back (or convinced someone else it was worth buying from you as a speculative asset). You might as well say money is destroyed when share prices tumble. Maybe this is the point of such arguments, to make the case that money is no different to any other asset, but we don't tend to treat it like that in reality. Or do we?
When that loan asset is written down the bank has to make up the difference from its equity - this ends up reducing the amount of loans it can write, so you get a contraction in the monetary supply.
So in the end, I guess the shorter answer is that a default destroys money in the same way that writing a loan creates it - you might well complain that no actual currency has been created or destroyed, but the argument is that it has a similar overall effect.