The rumors of Brock Lesnar being back in the USADA testing pool are indeed true, although the stories of meetings with the UFC are still being shot down as incorrect.
Any retired fighter who wants to fight again in the UFC must be part of the USADA testing pool for a minimum of four months. The last time Brock returned to the UFC last year, an exception was made and he was not subject to the four-month testing period. USADA still routinely tested him leading up to the fight and ultimately he failed twice.
Being part of this testing pool means that USADA officials will randomly visit Lesnar to test him for steroids or any illegal substance under the UFC rules. In 2016, Lesnar was tested eight times in total from the time he was announced as returning to fight day. In 2017 he was not tested and eventually pulled himself out of the testing pool in February.