if they deny it the only possible reason is to f--k foreign (in particular german) companies to favor US ones. sprint is practically dead now and t-mobile is doing fine but they would certainly be more competitive and would be able to offer a more competitive wireless market to consumers if they had deeper capital and a larger customer base for the transition to 5G.
John Legere is denying the story https://www.cnet.com/news/t-mobiles-john-legere-denies-justi... so we will see. The WSJ story could just be an attempt to sabotage the deal by introducing uncertainty.