Bellarmine's teaching on visibility, the basis for The teaching in Mystici Corporis, also states that occult or secret heretics are still members, joined by external union only, while catechumens are not members yet joined by internal union.
Mystici Corporis was correcting those who wrongly imagined a church with an invisible membership. This was later deviously exploited by the super-intelligent modernists, and misunderstood by the idiots who do their own theology and are simply looking for some proof texts for their own self-taught theories.