No, Augustine was clear what he believed the exception clause to mean. Divorce is justified because of infidelity, while remarriage is prohibited. As far as I have read, most Church fathers believed similar. I have yet to see one Church Father who explained the clause as meaning an unlawful marriage.
You are hung up on divorce being absolutely prohibited, which it’s technically not. Petrine and Pauline privileges tolerate it. And these Church fathers understood some divorce as separation without eligibility to remarry.
I personally believe the clause understood as unlawful marriage makes more sense, and/or including infidelity during betrothal (or non-consummated marriage). Deuteronomy 22 relates this infidelity as “fornication in her father’s house”.
The Church tolerates civil divorce because of some situations. She also tolerates separation because of infidelity. She also can decree a marriage invalid because of certain impediments.
If you’ve paid attention to anything I’ve said, I have not criticized any of that. I have simply questioned why the Church Fathers didnt see the exception clause as referring to invalid marriages. And i also recognize that leaders in Rome have criticized Tribunal interpretation and applications to cases.