This one is a few weeks old and it may not get much attention, but I am convinced that Paul was referring to a method of interpretation that was common in the first century. In the first century, Jewish interpreters would employ a method known as midrash.
The term midrash has been used in a variety of ways, but the essential element in midrash was the creation of fictional fables that were added to the interpretation of Biblical texts in the hopes of explaining the text.
In 1996 Charles Quarles wrote an excellent article entitled Midrash as Creative Historiography: Portrait of a Misnomer. It is available online, but it is somewhat technical: https://www.etsjets.org/files/JETS-PDFs/39/39-3/39-3-pp457-464_JETS.pdf
The article is mostly dealing with the concept that the gospels were written as midrash as an attempt to undermine the historical accuracy of the gospels.
Note the conclusion that Quarles makes regarding Midrash despite its current popularity:
While a rose by any other name may smell just as sweet, when the genre
midrash is given the more historically accurate label “Jewish myth” a
definite stench suddenly arises. When one moves beyond the slippery term
“midrash,” the genre that midrash critics portray as a “recognized and acceptable form of communication” to Matthew and his early Christian readers
This idea of midrash as Jewish Myth or Jewish fables is important when looking at Paul's reference in 1 Tim. 4:7 and elsewhere. William Mounce, in his commentary on the General Epistles mentions a number of passages that Paul was referring to the Jewish practice of creating fables as a way to interpret Scripture.
Mounce declares the following:
Repeatedly Paul calls the teaching “myths” (1 Tim 1:4; 4:7; 2 Tim 4:4; Titus 1:14). “Myths and genealogies” (1 Tim 1:4; Titus 3:9) are probably haggadic midrash: allegorical reinterpretations of the OT, perhaps as fanciful interpretations of the OT genealogies (Roloff, 64), especially of the patriarchs and their families. [bold added]
William D. Mounce, Pastoral Epistles, vol. 46, Word Biblical Commentary (Dallas: Word, Incorporated, 2000), lxix–lxx.
It is important to recognize that Paul was referring to fictional additions to the Biblical text and not a questioning of the actual text. In the question there appears to be an attempt to make an actual Biblical text (Exodus 7:10 ) into a myth. That is not the same as midrash and it is certainly not what Paul had in mind by the use of the phrase. He was referring to additions that were made outside the actual text of Scripture. To call Exodus 7:10 a silly myth is to reject the historicity of the Bible, which is a different category altogether.
As a practical matter this passage (1 Tim. 4:7) and the other Paul uses are a warning to those of us who preach and teach. It is a warning against the notion of allegorizing the Bible. It is also a warning when it comes to some of the illustrations we use as well. The illustrations can't be seen as additions that change the meaning of the original text, which can happen.