This person is expressing (although in deficient terms) one school of thought regarding the heretical pope question:
It's the Papa Haereticus ab Ecclesia deponendus school (a heretical Pope must be deposed by the Church).
And a different twist on the same question was put out there by Bishop Guerard des Lauriers in his sedeprivationist theology.
With both the deponendus school and the sedeprivationists, there IS in fact this notion of there being a material / juridical component to the papacy that needs to be addressed even in a heretical pope scenario. I wouldn't call it an "indelible mark" as if it were some kind of Sacramental character.
With the deponendus school, the papacy continues formally but in some kind of crippled state that needs to be addressed by the Church, whereas with the sedeprivationists, the papacy continues only materially, with the one consequence being that if, say, Jorge Bergoglio would convert back to Catholicism, he would formally take up the exercise of the papacy without any further action by the Church, since he retains the material designation.
Both these avoid the problem of "conclavism".
In short, the question is NOT as simple as what most SVs would have you believe. It's complicated by a question of needing AUTHORITY to make the determination of heresy and to establish the dogmatic fact of legitimacy.
St. Robert Bellarmine's position is way too simplistic.