Here is some more debugging: I think I do have a secret key matching the public key.
In order to check that, I have exported the secret key and the certificate in a folder, an then verified that the RSA moduli are the same:
% openssl x509 -inform der -in 'XXXXXXXX.cer' -noout -modulus
Modulus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
% openssl pkcs12 -in 'Apple Development: XXXXX.p12' -passin pass:foo -nocerts -nodes | openssl rsa -noout -modulus
Modulus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