I think DHH's perspective makes a lot of sense from the perspective of a web framework author. If you're going to buy into Rails it makes sense to buy in all the way, and not try to split your app logic across two very different languages and execution environments. Naturally he wants everything to be in Rails and for the DB to be treated as dumb as possible, it is always the way for a platform to want to subsume and abstract its lower layers.
In two-tier architecture there is no web framework, however, because there is no web server to begin with. All the UI logic runs client side and is written in the same language as the stored procedures. In this design there's nothing that's competing to be the place where app logic is implemented because you can't trust the client, so all trusted logic has to be in the database. In that case you buy into the RDBMS all the way, and really lean on its capabilities. The tension is resolved in a different direction.
In two-tier architecture there is no web framework, however, because there is no web server to begin with. All the UI logic runs client side and is written in the same language as the stored procedures. In this design there's nothing that's competing to be the place where app logic is implemented because you can't trust the client, so all trusted logic has to be in the database. In that case you buy into the RDBMS all the way, and really lean on its capabilities. The tension is resolved in a different direction.