That was just added by someone who's primary language probably isn't English. What they meant to say is "QPSK31, a mode that is related to PSK31, uses..."
Other than that, the claim "it uses more constellation points, that's a means error-correction coding", is plain wrong.
The error coding used is this, just one sentence later:
In this case, "allied mode" can be replace with "associated mode" or "kindred mode." The phrase indicates that there is a relationship between the two modes, BPSK and QPSK, without specifying the particulars of the relationship. "Allied mode" is not meant to convey or identify any specific technical content.