I have seen this "study" elsewhere I think, and I'm not very impressed with River's "science".
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(Edit) I've just seen crypto.com estimated 298 million for early 2024, so their numbers are quite different. Crypto.com estimates a 13% increase for 2024, which is low for a bullish year.
They're estimating 337 million users as of early 2025.
Of course that could be correct because it's a hint that Bitcoin's growth was fueled by institutional investors in 2024 and not by retail (as they also mention in the report).
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I agree with your opinion here because, looking at the current market capitalization and volume, it is enough to note that there is a sudden increase in the Bitcoin holding. This year alone, the institutional investors, as you have said, have contributed to the adoption and increase in the bitcoin market, putting more pressure on bitcoin purchases, which is a strong signal in the rise in bitcoin transactions globally. This also shows that the number of bitcoin transactions increased, and if it happens that such has occurred, then definitely the percentage of bitcoin holders would likely be on the rise as well.
According to the above statistics by CryptoPolitan, within six months, the bitcoin holders increased from 296,000,000 to 314,000,000, which indicates an additional 18,000,000 holders. Going by this, there must have been a 5x to 10x increase in the additional figure obtained within the six-month duration as stated. There is every possibility of an increase considering the fact that big organisations are buying more bitcoin, adding to their reserve, which triggered an increase in the price of bitcoin.