As someone who once believed this, this statement is simply not true. Experimentation in ham radio is largely about the physical layer, with maybe some layer 2-4 work when it comes to building low-scale communications infrastructure. Encryption is not implemented at this level, and the limitations on encrypted content do not impede research into new digital modes or actual radio science.

I think it is worth asking why modern digital radio standards (e.g. P25, DMR, LTE, 5G) were largely developed outside the ham community. My survey level understanding is they came from the communication needs of their users and are not a natural evolution of something that hams would build, with or without encryption. When I was researching DMR, there's a lot of stuff that is unnecessarily clunky when considering the use case of a ham QSO. Conversely, the business users of DMR want more structure than what's provided with amateur-designed digital modes. With 5G, I think there are some real technical and capital barriers, i.e., very few amateurs have access to the grade of test equipment needed to engineer and experiment with systems at those frequencies.