You have set up a chain of requirements beginning with Sections 501.10(A)(2) or (B)(2) with emphasis on the necessity rather than the convenience of a flexible connection. Then Section 300.11 plus the securing/supporting requirements of the rest of the associated wiring method such as RMC or IMC (usually Section 3xx.30) are applied along with Section(s) 501.140(B)(3)&(4). The general emphasis beyond securing/supporting the flexible coupling as part of the overall wiring method is avoiding stress on the connections/terminations.