The only way to prevent unemployment was to steer all of that idle industry to internal projects, causing the German economy to be hugely isolated (hence why you still see increasing GDP).
Either you are misunderstanding them or they are making a very elementary economic error. A country with a weak currency is a country where it is unusually profitable to export and unusually unprofitable to import. If a market economy is allowed to function (which was not the case in Nazi Germany) this allows for employment to increase for three reasons:
1) It is more profitable to hire people to make products for export
2) Foreign competitors to your firms have their prices artificially raised
3) Foreign investors in your country will get a better value for their investmetn
Note that all of these require a market economy to be operating without much obstruction. If foreign investment and exports are both being hamstrung, this wont happen.
But this still doesn't overcome the issues highlighted above. Yes German industry was attractive, but without the state intervention to keep the industry afloat, it would not have continued to expand.
Are you sure you read Tooze? Because a pretty core theme to Tooze is that shifting resources from one economic activity to another is just the mirage of total factor productivity acceleration, not the real deal. (He doesn't say total factor productivity but I think it's a useful term.)
Germany was a country that was ripe for investment, had plenty of entrepreneurs and was attractive to foreign capital. While there certainly were inefficiencies in the German market (the relatively low level of conglomerates), every economy has some problem or other and Germany was in pretty decent shape. Before the Great Depression their economy was doing fine so it should have done fine once the shock was past.