If it has a period of 5.4 days, then the estimation of mass may have been via the displacement of the red dwarf. If it were further out, like say a billion miles, then the planet's influence on the star would have been a lot less, since gravity follows an inverse square function.
Also keep in mind that the red dwarf has a much lower heat output than the Sun, thus it would not be like an object that would be orbiting 3.7 million miles from the Sun. Still it might be questionable if this is a gas giant like Neptune. At that distance would much of the atmosphere cook off? Maybe someone in that field more qualified to run the numbers.
I would speculate that this is a large mercury-like planet with no atmosphere. We still don't have enough precision in our measurements to account for earth-mass planets. There could be other planets in this system that could be in a habitable zone. It will be interesting to see what we find when we attain enough precision to find smaller planets around stars, I still think our stellar neighbors Alpha Centauri A&B will have some surprises.