My response is simple.
Pope Eugene IV approved the decree. Pope Pius IX (9) stated that only INVINCIBLE ignorance excuses one from the guilt of labouring in ignorance of the true religion (such as being in subjection to a heretical antipope). And the Church has always required abjuration of heresy or schism from converts to the Faith, when they were either publicly heretical or publicly schismatic.
In light of all these things:
Pius XI (11) was subject publicly to Benedict XV, who was a public heretic. He never abjured. He was publicly schismatic.
The fact that there was no uproar recorded proves nothing, other than that you haven't found evidence of it yet.
Interestingly enough, it seems that one Léon Bloy at one point denounced Benedict XV.