Neither Station X (at Bletchley Park) nor PC Bruno (at Vignolles, in France, where the Polish exiles were working) was reading Enigma traffic in February 1940. The first breaks into current Enigma were made in late March 1940. Naval Enigma was not read until mid-1941.
True in OTL but in ATL? In practice, It hardly matters either way in the case of the Altmark
(though it may for the wider purposes of the author of this ATL. We shall have to wait and see 🤔 )
In OTL, the RN had access to a few former prisoners of the Graf Spee who were released by that Captain when he entered Neutral safety
(unlike Altmark's, who broke these rules with the connivance of the Norwegians)
Moreover in this ATL, the RN has full access to many crew of the Graf Spee (they sank it at sea)
IMHO its highly likely that they will know that there are "many" prisoners on the Altmark, though perhaps not exact numbers.
NINJAed by said Author