You should read both what I wrote and what is written on the website you linked to.
I wrote:
"There [could] also be an issue if you have a warehouse in the US. You should discuss the details with a CPA.
If you only do dropshipping from China and have no employees in the US, then it should probably be fine, but I'm not a lawyer."
The website you linked to says:
"[...] the tax treaty may circumvent the US rights to tax you. In general, not taxable are:
Personal services performed from abroad
Selling digital products
Web design etc.
Selling physical products if the shipping point is from outside of the US."
It also goes on to state: "To date, we are not aware of any court ruling against this interpretation of the tax code [that Amazon FBA is not taxed in the US]. However, as with any aggressive approach to tax planning, there is always the possibility that the IRS eventually rejects it."
So not even they have heard of cases where people have had to pay US taxes in such cases. And that is with people using Amazon FBA (note I mentioned the warehouse above).
But OP is only talking about coaching and consulting services.
Nowhere in that article you linked to is it mentioned that by only selling to US customers, you are doing business in the US. Quite the contrary, it explicitly states that personal services performed from abroad are exempt from US taxes.
Even the IRS explains that such income is taxed where the services are performed, not where the customer is located:
https://www.irs.gov/individuals/international-taxpayers/nonresident-aliens-source-of-income