I was surprised by this and it turns out its a bit of a misrepresentation as you get unlimited fee free withdrawals if you meet one of these criteria which, in the UK, pretty much anyone except a teenager with no job will meet:
At least £500 was paid into a Monzo account in your name over the last rolling 35-day period, and you have at least one active Direct Debit on the same account in the same period.
You’ve received a Department of Work and Pensions or a Department for Communities’ payment into a Monzo account in your name over the last rolling 35-day period.
You’ve received a student loan payment into a Monzo account in your name over the last rolling 8-month period.
You’re sharing a Monzo Joint Account with someone who has done at least one of the above.
The criteria seem to be mostly designed to ensure that people who aren't using it as their main account can't do things that cost Monzo lots of money. That said, if you want to deal mostly with cash then Monzo probably isn't for you, and I think they're ok with that.
At least £500 was paid into a Monzo account in your name over the last rolling 35-day period, and you have at least one active Direct Debit on the same account in the same period.
You’ve received a Department of Work and Pensions or a Department for Communities’ payment into a Monzo account in your name over the last rolling 35-day period.
You’ve received a student loan payment into a Monzo account in your name over the last rolling 8-month period.
You’re sharing a Monzo Joint Account with someone who has done at least one of the above.