Not trying to defend Telegram.
While "CSAM" is the preferred term these days, it is not well-defined and not in any legal statute. CSAM includes some content that may be disgusting, but is not illegal. The only legal definitions in federal law are in 18 USC 2251, 2251a, 2252, 2252a, 2252b, and 2260. None of these laws mention "CSAM".