I don't know about that exact rule (most of my clients are not German). Honestly in my experience of living here and freelancing on this visa a lot of these requirements seem to be up to the discretion of the person doing the interview / checking your documents.
Edit: See here https://www.gesetze-im-internet.de/englisch_aufenthg/englisc...
(1) A foreigner may be granted a temporary residence permit for the purpose of self-employment if
1. an economic interest or a regional need applies,
2. the activity is expected to have positive effects on the economy and
3. personal capital on the part of the foreigner or a loan undertaking is available to realise the business idea.