Don't forget that tape has 4 tracks (used as 2x stereo in traditional audio tape). Other experiments have used 1 track as a pilot town to correct for flutter, but that still leaves 3x as much bandwidth as you were assuming.
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Found one of the ones I was thinking of. They were able to get about 24kbps on a single track, with a pilot tone on the other track to correct for flutter. This would imply 72kbps if using all 4 tracks simultaneously. At that rate, you probably get better quality audio with Opus than you would storing on a tape without some form of NR, though you only get half as much playback time.