Just like when Hitler let Allied troops be extracted from Dunkirk.
I just love how the media is uncritically repeating Nazi talking points. And this is the same soldier that the MSM has been showing giving updates on Mariupol. Note how he has the Azov patch on his chest covered. Previously he'd had it out in the open. Of course its all moot when there's another soldier right next to him proudly wearing their Azov patch on their sleeve.
Hitler admired the British and thought that them losing their army would result in widespread revolts in the colonies, resulting in the end of the British Empire. He thought this was a terrible outcome and would not allow it.
Remember, in 1940 the idea that the British would ally with the USSR was absolutely insane. As in, you'd be committed to an actual mental institution.
And then in 1941 Churchill excused it by saying "if Hitler invaded Hell I would allow a favorable word to be said about Satan in the House of Commons."