Why just market power? National security seems like it's also be a valid, defensible reason to kill a deal (it'd have been obvious to block Russian company taking control of a major US defense contractor, especially during the Cold War).
That's abusing the reasons major mergers like this are referred to the system in peacetime. It's not a hot war, it's not a cold ear, it's a trade war. What happened to the WTO? Did we all just walk off the reservation back to tarrifland?
Until that is rectified expect the WTO to not hold much sway over US trade policy.
The T-Mobile and Deutsche Telekom networks are heavily integrated. It would be disingenuous to say you can use vulnerable gear as long as it is outside US borders.