Virus-virus interactions are more than a theory at this point. They have been widely observed.
R0 is not a single number, it is dynamic over time (goes down as more people are infected). Further, the initial R0 for SARS-2 and influenza is not massively dissimilar. On top of that the rhinovirus data for UK, Sweden, Germany, and elsewhere show that the NPIs had little/no effect on the spread of that virus (similar to the case seen for adenoviruses).
That's logical issue #1: R0 is not that different for the two viruses, and the rhino- and adenoviruses spread as usual (same method of spread).
Logical issue #2: the established science according to agencies like the CDC and WHO were that such measures are not helpful for controlling the spread of influenza. This was borne out in an additional large number of published RCTs. Why are they suddenly effective?
https://wwwnc.cdc.gov/eid/article/26/5/19-0994_article
https://www.who.int/influenza/publications/public_health_measures/publication/en/
Logical issue #3: This is a virtual elimination of flu (and other betacoronaviruses) from wide circulation. Not so much a reduction. This has occurred everywhere, including places like India, and Sweden, Belarus, much of Africa, much of the United States, Brazil, etc. where things are carrying on w/ varying degrees of normality. How did NPIs, that didn't work before 2020 for flu, control the flu in places where they were scarcely carried employed?
Logical issue #4: Even if we go with the "R0 of SARS-2 is higher" concept, it also *supports* the virus-virus interaction theory. The more aggressive virus dominates, preventing proliferation of infection from influenza.
I'll also add that many scientists question the usefulness of R0. You can start here on that if interested:
