Is allowing both digitalSignature and keyEncipherment certificate usages a bad practice?
The guidance is more about keeping the certificates used for different operations seperate. Not keeping the certificates used for different ways of achieving the same operation seperate.

In the case of different TLS key exchange, the overall operation is still the same, i.e. Generating and exchanging a session key. While technically one method generates a key then sends it, with the other involves signatures they have the same overall lifecycle and consequence if the key linked to the certificate is disclosed.

The general guidance applies more to situations where the operation is different. E.g. With email signing there are two operations, encrypting the email (to prevent it being intercepted) and signing it (to prevent it being forged). As per this answer there are situations where you'd want to disclose the encryption key but not the signing key.

This doesn't apply to the different key exchange algorithms in TLS so the certificate usage is more of a technicality.

Of course if you then used the same certificate to sign something then that would be a terrible security practice.