If the C64 playback head is stereo (or reads one of the L/R tracks and ignores the other), that would likely work. But in this video he mentioned how he suspected that his audio tape deck failed him (it definitely records in stereo) while the data recorder that ultimately worked might be mono (in which case it might record a single track, twice as wide). Magnetic tape doesn't inherently have tracks until you lay some down with a record head.