The point is that Twitter charges their advertisers, their source of revenue, according to ad impressions served to "monetizable" users. They don't necessarily know at the time of serving the ad whether they're monetizable -- they might figure it out later. But advertisers are trusting them to not charge them for bots (or no more than 5% bots).

It could well be that 50% of accounts are bots, but as long as Twitter doesn't charge advertisers for them, they've been honest. But if their mDAU counts have been way off, advertisers will demand their money back.