I've read the Edwin Black book - he makes a reasonably compelling but nearly completely circumstantial case.

The case against IBM could be made against any multinational of the era, like Ford and General Motors. The fact that IBM's business was information processing does not add or subtract to their level of supposed conspiracy.

From the time of declaration of war (against the US), all american companies were hands off - even before that the relationship between Dehomag and IBM NY became more and more distant from 1933 on.

Beyond that, the technology was not held by IBM alone, Powers/Remington-Rand and Siemens and Halkse also had similar equipment and technology - so while IBM may have been the market leader, they were by no means alone - and choosing IBM over Powers in my mind is different than buying a Ford-werke truck over an Opel one.