The indult is not non-catholic. Therefore the canon law allowance doesn’t apply.
You're not one for distinctions, are you?
1) Indult (= Tridentine Mass) is most certainly a Catholic rite (despite your broad equivocal use of the term "catholic")
2) Those who offer the Indult profess the Catholic faith. While they may or may not adhere to various errors, they are most certainly not in the same category as a declared formally-schismatic "church", such as the Orthodox. Even the most dogmatic SVs like Bishop Sanborn acknowledge this distinction, stating that for this reason they are not required to make abjurations before becoming Traditional Catholics.
While, objectivley speaking, the Conciliar Church is a (non-declared) non-Catholic institution, the Mass is Catholic and those who offer the Mass (and assist at it) do in fact profess Catholicism.
Now, one is permitted to passively attend even DECLARED non-Catholic funerals / weddings (all other considerations aside, such as risk of scandal or corruption of your faith -- and the risk of scandal is almost non-existent these days, since NO ONE construes attendance at a wedding or funeral as an endorsement of the religion under the auspices of which it's taking place).
Then,
a fortiori, one may certainly attend the Indult Mass, and it is my opinion, even actively assist at it, since it's a Catholic Rite offered by those who profess to be Catholic (prescinding now from considerations regarding validity ... so, let's assume for the sake of argument that it's being offered by an elderly priest ordained before the New Rite(s) or by an ex-SSPX priest).