I didn't say incapable of producing a heavy four engined bomber but rather incapable of mass producing or even series producing one in sufficent quantity to make it useful in service.
A single Me 264 was built and flown a handful of times. Junkers produced the Ju 290 at a rate of one or two a month. The He 277 is another single prototype and the He 177 had a production run of just over 1000 in about 3 years. Focke Wulf produced a few hundred Fw 200s.
At the same time, the US was producing several B-24s and B-17s an hour and England likewise was producing Lancasters, Sterlings, Halifax etc at a rate of dozens a day.