When the deafult SunX509KeyManagerImpl is being used we are in violation of TLSv1.3 RFC spec because we ignore peer supported certificate signatures sent to us in "signature_algorithms"/"signature_algorithms_cert" extensions:
https://datatracker.ietf.org/doc/html/rfc8446#section-4.4.2.2
https://datatracker.ietf.org/doc/html/rfc8446#section-4.4.2.3

X509KeyManagerImpl on the other hand includes the algorithms sent by the peer in "signature_algorithms_cert" extension (or in "signature_algorithms" extension when "signature_algorithms_cert" extension isn't present) in the algorithm constraints being checked.