For me the main and obvious thing about DAU is that the majority of users are readers-only. They don’t tweet or like stuff. Firehose might help with detecting non-bot _tweeters_.
Pew analysis (based on surveying humans) says "97% of tweet volume comes from the top 25% most active tweeters"[1]
So Musk is arguing about a statistic that twitter claims, which is that spambots make up less than 5% of users.
Now, if those bots are part of the most active 25%, then they make up 1/5 of the 'most active' user population. So it seems plausible that they could well be responsible for 1/5 of tweets.
Which matches exactly what Musk is claiming - that 20% of tweets are from spam bots.
Which still makes no difference to Twitter's ability to sell Advertising targeting the 95% of users who are humans, 75/95 (=79%) of which are only responsible for 3% of tweets, but... presumably... 79% of ad revenue. Because they only serve you ads when you're looking at a feed. And that's what those users are using Twitter to do.
[1] https://www.fastcompany.com/90697826/the-top-25-most-active-...