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I was obvious to me what Struthio meant. When he said a manifest heretic "may not be a heretic," he meant a public heretic who may not have the pertinacity of will to be culpable as a formal heretic, and may not be a heretic internally if you will, but only exteriorly and publicly.

If you want, I can quote you Van Nort and probably other theologians who indicate that a public, material heretic is considered outside the Church even if lacking the pertinacity of will of formal heresy is lacking, thus in a very real sense showing a manifest heretic who is yet not a "heretic" in a meaningful sense.

Van Noort, Dogmatic Theology Volume II: Christ's Church, p. 241-242

 b. Public heretics (and a fortiori, apostates) are not members of the Church.  They are not members because they separate themselves from the unity of Catholic faith and from the external profession of that faith. Obviously, therefore, they lack one of three factors—baptism, profession of the same faith, union with the hierarchy—pointed out by Pius XII as requisite for membership in the Church. The same pontiff has explicitly pointed out that, unlike other sins, heresy, schism, and apostasy automatically sever a man from the Church. "For not every sin, however grave and enormous it be, is such as to sever a man automatically from the Body of the Church, as does schism or heresy or apostasy" (MCC 30; italics ours).

    By the term public heretics at this point we mean all who externally deny a truth (for example Mary's Divine Maternity), or several truths of divine and Catholic faith, regardless of whether the one denying does so ignorantly and innocently (a merely material heretic), or willfully and guiltily (a formal heretic). It is certain that public, formal heretics are severed from the Church membership. It is the more common opinion that public, material heretics are likewise excluded from membership. Theological reasoning for this opinion is quite strong: if public material heretics remained members of the Church, the visibility and unity of Christ's Church would perish. If these purely material heretics were considered members of the Catholic Church in the strict sense of the term, how would one ever locate the "Catholic Church"? How would the Church be one body? How would it profess one faith? Where would be its visibility? Where its unity? For these and other reasons we find it difficult to see any intrinsic probability to the opinion which would allow for public heretics, in good faith, remaining members of the Church.

b) That merely material heretics, even if manifest, are members of the Church, is argued by Franzelin, De Groot, D’Herbigny, Caperan, Terrien, and a few others. But the contrary opinion is more common.[13]

[13] Those who include material heretics, even if manifest, in the Church: Franzelin, Theses de Ecclesia Christi, th. 23, pp. 402-423; J. V. de Groot, De Ecclesia, q. 8, a. 3; D’Herbigny, n. 355; L. Caperan, Le problème du salut des infidels (1912); J. B. Terrien, La grâce et la gloire I (1901) 330.

https://lumenscholasticuм.wordpress.com/2016/12/05/fr-salaverri-on-whether-heretics-apostates-schismatics-and-excommunicates-are-members-of-the-church/#_ftn13