The Roni itself is free to purchase and not under any regulations.
However, once you put a Glock in it, it becomes a "Handfeuerwaffe" (i.e. a rifle), because it is "usually fired from a shoulder stock". This definition is stated in the law.
Now, since you now have a rifle at your hands, the regulations regarding magazine capacity come into play. The Glock has a capacity of greater than 10 rounds, so the rifle at your hands falls under the exception permits (ABK).
BUT if the Glock was bought before August 2019, the "grandfathering" applies and you're fine to use it. If you'd have bought the Glock later with a normal permit (WES), you'd commit a crime. If you bought it later with an ABK, you're fine.
The length of the Roni us irrelevant since it is below 60cm both folded and unfolded so that regulation does not apply.
Also as said the Roni itself is irrelevant, it all comes down to how and when the Glock in it was obtained.