"why would another tax preparation practitioner advise so?"

Because someone heard something that was wrong or stated wrongly, or heard it incorrectly, or (excuse me, folks...) they are a Practitioner because they are only Practicing and don't know the rules that apply.

I find the terminology helps.

A single-person LLC is a Disregarded Entity = doesn't exist outside of that Person. This is treated as if they are operating as a Sole Proprietorship and file the business info on their own 1040 using Sched C.

W3 is Payroll. It is a Summary for all W2 issued. Were there W2 issued, to anyone?

Let's review that "sole proprietor" and "single-member LLC" don't get paid through Payroll, because there is no payroll for that person.

So, do we suspect a mistake has been made, or just bad guidance given?