Whenever you see this, it is almost always due to a deleted answer.
In this case, the user wrote an answer to the question, but then deleted it him/herself immediately.
When you hit 10k rep, you'll be able to see deleted answers on questions.
I question your premise of:
[...] that questions about sole proprietorships are on topic.
That's not completely right. Rather, small business and self-employment questions — including sole proprietorships — are on topic when they relate to the owner individual's personal income or compensation from the business, including how taxes (at any ...
In my opinion, this type of question is on-topic here.
According to our on-topic help page, questions about sole proprietorships are generally on-topic. An example sub-category that is explicitly mentioned as on-topic is "When to incorporate to protect your work or assets." It follows that the pros and cons of incorporating your sole-proprietorship as an ...