No. Secrecy will have backdoor keys, but that is not what walled garden means: it means more like people have no power over the decisions made in the garden.
"... those designated as gatekeepers will have to:
allow third parties to inter-operate with their own services, meaning that smaller platforms will be able to request that dominant messaging platforms enable their users to exchange messages, send voice messages or files across messaging apps. ..."
Thank you, I should have clarified that in the original comment.
(I admit that I actually edited that original comment, since the link I initially included was more speculative, talking about a law that was expected to pass, and being much more vague about when it might enter into force. The page at the updated link is more definitive, but not as narrowly focused, so I'm glad you managed to isolate the relevant section.)
"... those designated as gatekeepers will have to: allow third parties to inter-operate with their own services, meaning that smaller platforms will be able to request that dominant messaging platforms enable their users to exchange messages, send voice messages or files across messaging apps. ..."