I share your instinct. That's why I wrote this in the paper:

Taken together, the results are both surprising and unsurprising. They are unsurprising in the sense that they conform with economic theory: demand curves slope downwards (when posting costs go up, number of posts goes down), and signaling theory works (when posting costs go up, higher quality posts are made). They are surprising in the sense that even such small micro-incentives (the average posting cost is just 51 sats, or about 5 cents) are enough to influence user behavior in such a way that post quality is improved. The results suggest that pay-to-post may be an effective mechanism for mediating content quality, even at very small monetary amounts. Moreover, the assumptions required for this result to hold more broadly are fairly weak: we require only that expected rewards are increasing in post quality (Assumption 3, that post quality and intrinsic motivation to post are positively affiliated (Assumption 2), and that extreme levels of intrinsic motivation are sufficiently rare (Assumption 1). All three assumptions are likely to be satisfied in other social media environments, and thus it may be possible to extrapolate our results to other settings (though perhaps with different baseline cost and rewards levels.)