The Yugoslavian regime (unlike other communist regimes in Eastern Europe) tolerated religion.

There were therefore (in the Catholic world) bishops, priests, friars, nuns, parishes, catechisms, religious books (including those on Marian apparitions, e.g., Bozidar Nagy, Lurd: susret neba i zemlije  [Lourdes: Where Heaven Meets Earth] Zagreb, 1979; Josip Sukner, Veliki znak. Ukazanja i poruke Presvete Djevice [The great Sign. Apparitions and messages of the Blessed Virgin], Zagreb, 1975; religious periodicals with circulation numbers in the tens of thousands (e.g., Glas Koncila [The Voice of the Council]).

The regime even built new churches. Saverio Gaeta himself, recalling the history of the church of St. James of Medjugorie, reminds his readers that it was completed in the late 1960s and early 1970s, and that "three sister churches were also built during the same period: one in Šurmanci in 1964, one in Vionica in 1969 and one in Miletina in 1970" [S. Gaeta, Medjugorje. La vera storia, San Paolo, Cinisello Balsamo, 2020, p. 6, n. 2].

Saverio Gaeta believes the "visionaries'" statements and writes:

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Corvaglia argued, "In Medjugorie, rosaries were prayed regularly in all families, and people went to mass every Sunday, but they had never heard of Lourdes and Fatima? Who would believe that?" But let's read what his colleague Bouflet [a critic of Medjugorje, ed.] writes, "A survey conducted among 300 people reveals that the name Medjugorie means nothing to 213 of them."
If it's feasible that in democratic France, more than two thirds of these respondents were unaware of the existence of Medjugorie after a good seventeen years of apparitions, then how can it be said with certainty that in 1981, in a remote village in communist Yugoslavia, did six teenagers have to know about Lourdes and Fatima?
[Ibid., pp. 33-34]

To begin with, it was not necessary for all six to know about Lourdes or Fatima (one or a few of them would have been enough), but let's let Father Jozo (the parish priest of Medjugorie at the time the "apparitions" began) respond.

Recalling his first conversation with "visionary" Mirjana (which took place three days after the first "apparition"), he says in the interview-book with Sabrina Cović:

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I was surprised that she had never heard of Lourdes or Fatima, and I lent her a book translated into Croatian for her to read.
[Sabrina Covic, Incontri con Padre Jozo, Sakramento, Grude, 2006, pp. 53-54]

If Father Jozo was surprised, this means it was normal that a Yugoslavian Catholic teenager would know about Lourdes and Fatima.

Therefore, if, after becoming a "visionary", a Yugoslavian Catholic teenager says he or she didn't know about Lourdes or Fatima, it's suspicious.
If more Yugoslavian Catholic teenagers, after becoming "visionaries", say they didn't know about Lourdes or Fatima, it's extremely suspicious.
It's simple.

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The statistical and probabilistic comparison provided by Saverio Gaeta between the (actual) lack of knowledge of Medjugorie in France and the (hypothetical) lack of knowledge of Lourdes and Fatima in the former Yugoslavia, also cannot stand, for logical reasons.

To begin with, it's obvious that Lourdes and Fatima have always been much better known than Medjugorie. If we really feel the need to resort to numerical data, just use Google Trends (the tool that provides data on searches performed by users in Google's search engine). These are the results for worldwide searches from January 1, 2004 (first available date) to June 30, 2020:

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