He lashed out on Twitter and took the unusual step of threatening a lawsuit over the result, a move that might be called dubious in the same way that LeBron James might be called athletic. It is — but that’s underselling the point a bit.
It is trivially obvious that LeBron James, a great athlete, is athletic. Thus, this comparison makes the point that it is trivially obvious that the lawsuit in question is dubious, meaning not justified. "Underselling" is equivalent to "understating", not saying something strongly enough. So, the lawsuit is worse than dubious, it's ridiculously unjustified.