However, I might interpret someone in payroll as a key worker (and typically, payroll will be done by a bookkeeper and not an accountant).
The post refers to a self employed accountant working from home. That suggests a small business accountant. Such people usually do everything, book-keeping, payroll, accounts, tax returns. It's only really bigger firms with several staff where you'll have book-keepers doing the payroll etc.
I'm a fully qualified accountant, working on my own, and have been an accountant for 37 years. I can spend day after day just running payrolls at some times of the month. Added into that, at present, I also have to make the furlough claims for which we only have a 14 day period, i.e. December claims must be made by 14 January. That puts an enormous amount of pressure on just a few days.
If I didn't do payrolls, I wouldn't have the clients for the other work, such as accounts and tax returns. Small business clients, sole traders, etc., usually want a "one stop shop", a single person or firm to do everything for them. If I only did accounts and tax returns, they wouldn't engage a different book-keeper to do the payroll - they'd engage someone else to do the whole job, they don't want to be faffing around with different people doing different parts of the job.