I think that it would be very wrong to say that the only reason that Hitler invaded Poland was because of Polish military mobilization. I think you could say that it was one of the reasons, but certainly not the only one. Hitler mainly wanted to just expand his empire.
No, Erika Steinbach's contention that Hitler invaded Poland because of Polish military mobilization is not accurate, because Hitler invaded many other countries whose military was not mobilized. Hitler just wanted land. Hitler hated the Jews and wanted to find any way that he could to destroy them. Poland represented the things that he hated.
When the Germans invaded Poland in Sept. of 1939 it was under the guise of a Polish border incursion which was totally fabricated by the German Army. The reason Germany invaded Poland was to expand the size of the empire of the third reich and reclaim german territories lost in the first world war.
Erika Steinbach is not exactly right as to the reasons that Hitler invaded Poland. The fact that Hitler had signed a nonagression pact with Poland made a lot of German people unhappy because Germany had lost a lot of people to Poland in WWI. It seems to be a revenge move on Hitler's part.
I think Hitler would have invaded Poland for one reason or the next, so blaming it on one specific action is not useful. Many people tried many different tactics to stop Hitler's domination of Europe, but most were not effective. Ultimately, I believe Hitler invaded Poland simply because he knew he could.