> I find this surprisingly specific. A help book on the source language specific to the target language... What part of English grammar is only important to students of German but not students of Dutch or Polish?

Quite a lot, since even though English is a very strange Germanic language it is at its core in many ways still Germanic, so it has a large overlap with German that it does not at all share with Slavic languages like Polish. It also shares a lot with Danish (which I find to often sit somewhere in the middle between English and German) and I presume Dutch as well. So yes, that book might very well be helpful in learning Dutch too, it is just a way smaller niche.

Source: I could be considered a native speaker of both German and Polish.