Greetings … Well, this preliminary marking on the 9th symphony could also indicate that Beethoven thought other people might be unsure of how to operate the thing. Musical notation is intended for communication, which implies two (at least) people. If the potential for confusion exists, it does not imply Beethoven himself was confused, let alone systematically mistaken.
Indeed the Maelzel marking is potentially misreadable by non-experts - though my piano teacher never had a moment of doubt that it should be taken from the top of the weight - and the original machines were definitely easier to read from the top, see Johann Nepomuk Maelzel - Wikipedia).
It seems a stretch to assume that Beethoven, being alive at the same time as Maelzel, meeting him in Vienna and having had a large degree of communication with him, and becoming as a consequence a quite strong advocate of the metronome, would have made this basic mistake throughout nearly a whole decade without there being some tangible record that he had made a mistake.
For example, there might be an entry in his conversation books saying ‘Damn that Maelzel, I have been taking his tempo from the bottom of the weight all this time when it should have been the top! Now I will have to get Steiner to re-engrave all the pages with those stupid numbers. But at least I can get it right for my 9th symphony!’.
But, there is no such evidence of Beethoven being aware of making a mistake.
The ‘clincher’ in this context is … that 108, as a tempo for the 9th symphony first movement, is also a lot faster than almost all 20th century conductors take it. Even ‘period’ experts. Probably even more than 20% faster.
So the theory utterly fails to explain the case it ought to explain perfectly : the discrepancy between Beethoven’s marked tempo (108bpm) and the ‘usual’ tempo for the very movement where this marking appears (70-80-ish). Norrington’s first movement at only quarter=82 already sounds fast enough to elicit objections. Even the posthumous consensus marking quarter=88 is ‘too fast’ relative to almost all recordings.
The ‘from the top’ tempo 108 is, though, consistent with his other ‘too fast’ tempos in being … well, too fast. Just like the Hammerklavier first movement. Whatever one may think of the ‘108 or 120’ marking it doesn’t do anything to reconcile Beethoven’s numbers with 20th century performance.
Beethoven somehow - despite knowing Maelzel personally - screwing up how to read the metronome is a ‘clever’ proposal that has been made several times, but it turns out to be superficial, illusory, and not a solution to anything. There are still possible psychological explanations for ‘too fast’ markings - eg the difference in perception between playing a piece ‘in one’s head’ or on the piano, vs. it being performed by a full orchestra in a hall - and we will have to be satisfied with suchlike for the time being.
(Much the same could be said for Schumann’s markings.)
Some of the same points are made in a nice article here - metronome marks for Beethoven’s Ninth Symphony in context | Early Music | Oxford Academic