As indicated in a prior thread on this site, there is good reason to doubt the authenticity of the Zohar, i.e., that it is attributable to Rabbi Shimon Bar Yochai.
Question: if that is the case, does it logically follow that all those people (including great Rabbis such as the Vilna Gaon) who spent much time studying the Zohar and even writing commentaries on it were engaged in bitul Torah during such time, or at least cannot be said to having been engaged in Talmud Torah?
It seems like this could be comparable to the Braisa that mentions the methodology of Shimon HaAmsuni (can be found in Pesachim 22b, among other places):
כדתניא שמעון העמסוני ואמרי לה נחמיה העמסוני היה דורש כל אתים שבתורה כיון שהגיע לאת ה׳ אלהיך תירא פירש אמרו לו תלמידיו רבי כל אתים שדרשת מה תהא עליהן אמר להם כשם שקבלתי שכר על הדרישה כך אני מקבל שכר על הפרישה עד שבא רבי עקיבא ודרש את ה׳ אלהיך תירא לרבות תלמידי חכמים
As it was taught in a baraita: Shimon HaAmmassoni, and some say that it was Neḥemya HaAmmassoni, would interpret all occurrences of the word et in the Torah, deriving additional halakhot with regard to the particular subject matter. Once he reached the verse: “You shall be in awe of [et] the Lord your God; you shall serve Him; and to Him you shall cleave, and by His name you shall swear” (Deuteronomy 10:20), he withdrew from this method of exposition, as how could one add to God Himself? His students said to him: Rabbi, what will be with all the etim that you interpreted until now? He said to them: Just as I received reward for the interpretation, so I shall receive reward for my withdrawal from using this method of exposition. The word et in this verse was not explained until Rabbi Akiva came and expounded: “You shall be in awe of [et] the Lord your God”: The word et comes to include Torah scholars, and one is commanded to fear them just as one fears God. In any case, Shimon HaAmmassoni no longer derived additional halakhot from the word et.
The implication from Shimon/Nechemya HaAmsuni's statement (before Rabbi Akiva's addition) seems to say that since the expounding of the word 'Es' was done in good faith*, even though he later concluded that it was all in error, he believed he would still receive reward for learning Torah.
Since presumably the Vilna Gaon and others had a sincere and founded belief that the Zohar is a legitimate Torah work, it follows that they too would be credited as having done Talmud Torah, even if ultimately it ends up that the Zohar is proven to not be a 'legitimate' work.
*I'm taking it as a logical assumption that all parties involved acted in good faith. While it's probably beyond the scope of this answer to define what learning in good faith is, I would guess it's something like the scholars reached their conclusions based on legitimate and accepted methods of determining the proper way to learn.
No, it doesn't follow logically. At the very least, your syllogism is missing the step "if it wasn't authentically written by RSBY then it's not Torah".
