A myth? Well, no, as French blog Osservatore Vaticano reminds us today of yet another example:
This past Saturday, users of the Liturgy of the Hours promulgated by Paul VI said at Vespers, perhaps without knowing it, a text personally composed by Max Thurian, who was, at that time, a Calvinist minister [he would enter the Church almost two decades later, in 1988]. According to his own testimony, the Geneva native, who was an "observer" representative of the Taizé community on the commission guided by Monsignor Bugnini, personally wrote the second prex (intercession) of Vespers for the Feast of the Transfiguration.
Afterwards, writing on the columns of Notitiae, the periodical of the Congregation for Divine Worship, he complained of the "liberties" taken by the translators [of the vernacular versions of the Liturgy of the Hours] regarding his text (Notitiae n°171, Oct. 1980, p. 506).
There is, of course, an easy solution for all who wish to avoid Protestant-composed literature in the Divine Office: it is called the Roman Breviary.