It seems like there was probably a degree of duress and that this was a continuation of a series of "interrogations" where perhaps a greater deal of duress was necessary? Not that the German scientists were necessarily unwilling but that they were anyway required to attend:
>"As the behaviour towards German scientists had already improved considerably in 1947,the interrogation had the more elegant form of a small colloquium. A handful of German specialists from Göttingen had been invited among them also myself. The significant German people who there gave talks were Professor Alwin Walther who had been thoroughly concerned with Hollerith machines and differential analyzers at the Darmstadt Institute of Technology, and Konrad Zuse with his relay calculators. If in addition Professor Friedrich Willers from Dresden had also participated, all leading German scientists who have been busy during the war in the development of sequence controlled calculating machines would have been there. Though Willers was in the Soviet occupation zone and therefore probably not available for the Britons." (http://www.mathcomp.leeds.ac.uk/turing2012/Images/Turing_Zus...) //
Note that Porter from the NPL later mentions performing an "investigation" as part of a military operation, in his military uniform (despite there being no requirement to do so). Also:
>"Undoubtedly, the participation at the colloquium of Göttingen was not optional. (ibid, p.5)"
tl;dr - It sounds like there's evidence to suggest Turing had an unreported meeting with Zuse in 1948. There is also a suspicion that Turing could have met with Zuse in 1934 on a trip to Gottingen.