In your question, you may not understand the concepts of branch and subsidiary. A subsidiary is a real company, which has the legal personality of obtaining the business license of enterprise legal person and has its own independent business scope.
The reply of SAIC that "subsidiaries can't undertake real estate development projects" may refer to the situation that parent companies can't subcontract with each other in the national procurement law or bidding law, which has nothing to do with the company's business scope.