The law says "[the Secretary] may waive or modify any statutory or regulatory provision applicable to the student financial assistance programs under title IV of the [Education Act] as the Secretary deems necessary in connection with a war or other military operation or national emergency."
It doesn't say that the Secretary can cancel the debt entirely. I think it's somewhat of a stretch to say that waiving or modifying a regulatory provision related to the debt is the same thing as canceling it. Most people would interpret that as meaning something like you can pause repayment, adjust interest rates, etc.
It doesn't say waiving or modifying "a" regulatory provision, it says waiving or modifying "any" regulatory provision related to the debt. That certainly includes canceling it. "Any" and "waive" aren't up to reasonable interpretation here, which is probably why Roberts dodged them in favor of drilling down exclusively on the word "modify" and taking his textualist hat off so he could ignore the law as written and approved by Congress can be ignored because it had never been used in identical fashion before by a president.
Two small silver linings though. Roberts and his majority pulling all of these crazy stunts makes the SC much more interesting and has generated a ton of public interest. Public awareness and participation is gonna be an important step in reforming the court. Second, now that the majority no longer bothers with creating a legal pretext for its edicts (they aren't decisions without actual standing), they have created space within the legal world for serious conversation and effort towards reforming the SC to return it to its original intent as a judicial rather than legislative body. Reading some of the dissenting opionions it should strike anyone that the veneer of legal plausible deniability behind the conservative court's actions has been pierced to the point where you have dissenting justices quite explicitly calling out the unconstitutionality of the court's actions in writing.
"In adjudicating Missouri’s claim, the majority reaches out to decide a matter it has no business deciding. It blows through a constitutional guardrail intended to keep courts acting like courts," she wrote, adding that by deciding the case, the high court "exercises authority it does not have. It violates the Constitution."
It doesn't say that the Secretary can cancel the debt entirely. I think it's somewhat of a stretch to say that waiving or modifying a regulatory provision related to the debt is the same thing as canceling it. Most people would interpret that as meaning something like you can pause repayment, adjust interest rates, etc.