My guess would be that there's some industrial process that completely separates any yeast from anything else so as to remove any potential hop bitterness. And that there's also likely been some updates to the process over the years. It may have started out as boiling things down in the 1920s, but may have moved on to an enzyme catalyzed process these days?
On the idea of using a stout: you will end up with even more bitterness than the version you made, more than likely. Stouts (especially higher alcohol stouts) tend to be fairly generously hopped compared to a standard light lager, in order to balance the sweetness of the added malt. You just don't taste them as being super hoppy because a) the hops are added early in the boil for bittering and most of the volatile aromas boil off, and b) because, well, they're doing their job of balancing the sweet malts to make the beer not taste sickly sweet.