A lot of people today overestimated the reach and destruction that resulted during the C.R, the C.R movement was basically the reflected version of the January 6th riot in the capital multiplied 5000-fold, with Mao Zedong in lieu of Donald Trump; not everyone in society or in the CCP government agreed with them. You got people high up in the echelon like Zhou Enlai who ordered for PLA soldiers to guard the Forbidden City from the Red Guards (the reason why the Forbidden City & other religious buildings and monuments still exist in China today)

In addition, the destruction of the C.R happened most commonly in the big towns and famous cities, whilst most of the poor, remote and rural towns and villages were far away from the chaos, and it is there that the majority of the old information, cultural norms and customs still managed to survive to be passed on, TCM included. The people inhabiting these places would be more focused on agriculture, putting food on their table and not starving to death than wasting their time on book and statue burnings. The circumstances of poverty and remoteness of these Chinese villages and towns would have ensured the survival of TCM practices considering these places probably still did not have anything remotely close to infrastructure or public health care.

EDIT: Also included Chinese diaspora who fled to other countries with cultural and traditional customs and knowledge then returned back once the C.R died down, also ensuring its survival.