It is a common device by Polish commentators to claim that any Polish citizen who actively collaborated with the German occupiers was an "ethnic German". For example, it is claimed that the mayor of Jedwabne, who led the pogrom against the Jewish residents of that town in 1941, was actually an 'ethnic German".
The reason for such claims is quite clear; it is to present the idea that no true Pole, a person of solely Polish descent who identified fully with the Polish people and its State, could possibly have stooped so low as to collaborate with either the German or Soviet occupiers, unless he were a member of a criminal underclass. In that view, only members of the ethnic minorities, Germans, Ukrainians, Jews (in the Soviet-occupied zone), non-Polish people considered even before the war as hostile and disloyal to the Polish State, could collaborate in such a way, the corollary being that collaboration was in itself an indication of being a member of one of the non-Polish minorities, either openly or secretly.
The problem with that interpretation is that the whole concept of "ethnic German" was a very hazy one, given the centuries of ethnic mixing in the German-Polish borderlands. In the German-occupied part of Poland it was the German authorities who determined which parts of the population under their control would receive the official designation of "ethnic German" and the privileges and duties that went with it, and willingness to collaborate was a major criterion in making that determination.
Thus, the Reichsstatthalter of Reichsgau Danzig-Westpreussen, Albert Forster, arbitrarily declared as "ethnic German" all the native inhabitants of that Gau, ie excluding Poles who had settled there after 1920, when that former German territory had come under Polish sovereignty.
An example of the fluidity of the concept of "ethnic German" is the experience of certain Polish citizens who were granted the status of "ethnic German" and subsequently conscripted into the Wehrmacht. A number of such men went into Allied captivity at the surrender of the Axis forces in Tunisia, and subsequently declared themselves to be really Poles, as a result of which they were allowed to join the Polish units that were part of the British Army. Thu8s, those men were "ethnic Germans" while serving in the Wehrmacht, but "Poles" while serving in the Polish units.
As far the photograph at issue is concerned, it may be that all the men in Polish uniform shown in it were genuine ethnic German citizens of Poland, but it is equally possible that some of them had not previously identified themselves as such, but had received the status of "ethnic German" as a result of their willingness to collaborate. The fact that the men were designated in the caption of the photo as "Volksdeutsche" is not itself conclusive; there would have been a clear incentive for Polish POWs to claim "ethnic German" status in order to be released from captivity.
What is historically true is that the German occupiers did not create any military units that were officially designated as Polish, in the same way as there were units designated as Ukrainian, or Latvian, or Estonian, or French. But the Ukrainian SS-Division "Halychyna" consisted of men who until 1939 had been Polish citizens, and given the rather hazy distinction between ethnic Poles and ethnic Ukrainians in the Galician region it is entirely possible that some of the men who were recruited into the "Halychyna" Division had previously had a Polish ethnic identity.
The bottom line is that must have been a reasonably large number of persons serving in various German military units who were ethnically Polish, although they served under different ethnic designations, eg ethnic German, ethnic Ukrainian, ethnic Russian, ethnic Latvian, etc.