It’s totally at the discretion of the bank. It seems that Monzo has changed its T&Cs sometime between contract terms v1.7 on 28th March 2019 and v1.9 on 23rd January 2020. V1.8, whilst linked, is peculiarly unavailable on the Monzo website.
According to Monzo T&Cs they don’t want people using the personal account for business purposes. They don’t define what they mean by business or whether that includes sole traders.
Bear in mind that, as a sole trader, there is no actual requirement to even have a separate account from your own, but I’d suggest having a dedicated account will likely make your life easier in accounting terms.
Monzo is desperate to monetise since it has received an awful lot of investment but isn’t making a profit, so it stands to reason that they’d direct sole traders to use the business account (and pay for the privilege). Other banks may allow you to use personal current accounts for non-Ltd Co. Business use, ie sole trader.