No, it's not "but an allegation", it's been confirmed by numerous legal sources and never once refuted by CB or his legal team.
Regarding MWT's bafflement of why he hasn't been prosecuted over the specific haul from the box factory, that seems down to yet more poor research on the part of MWT. HCW has already explained it is due to the double jeopardy law because CB had already been prosecuted for possesion of CP files when they searched his computer in 2014. This was after his ex-gf reported him for abusing her daughter and he was prosecuted for that offence as well as for other CP content on his devices.
From JC's interview with HCW:
‘But they were the force that got the search warrant in Neuwegersleben, where they did indeed find child *advertiser censored*.’
This was where the problems began, because among the 8,000-plus images and videos they found in early 2016 was the series of Anja P’s daughter. ‘They were the same files we used to prosecute him over and that is why he could not be sentenced again. In Germany, if you get charged for one thing, you can’t be sentenced for the same thing.
‘It was basically impossible for us to prove that when his flat was searched in Braunschweig he didn’t already also have all those photos in Neuwegersleben. So we ended up only charging him for a small proportion of the material he already had.’ He admitted Brueckner had been ‘incredibly lucky’ as if he had not already been sentenced in Braunschweig on the far fewer files, ‘we would have been able to sentence him for all these files in Neuwegersleben. He would have got a higher sentence.’
From what HCW explains, it sounds like police could not date any of the files as being acquired by CB post 2014 and therefore, it is effectively the same offence for which he was already convicted, albeit missing a large part of the evidence which (if it was discovered at the time) would have likely yielded him a longer sentence.