Maybe the way they do it is by reducing the mass of the parts...A bit. To be honest hearing about thrust-to-weight ratio in a turbofan of 1:20 seems kind of mythical or nonsensical. Kinda like Intel's roadmap to having 10 GHz processors by 2010. Some kind of pie in the sky goal. That would likely require using denser fuel and an alternate engine cycle. I doubt you can achieve it with a regular turbofan. Maybe something like the SABRE engine would work. You use some sort of cryogenic fuel like liquid methane to chill incoming air to liquefy it and then you burn that. Outside of rocket engines thrust-to-weight ratios like that are unheard of.
I doubt you can get to a thrust-to-weight ratio of 20 with a turbofan. Simply too many parts. I sincerely even doubt something like that is all that useful either. Human pilots would hit G limits if the acceleration was too high.