While a 5k might not be able to defeat a 30k. A 50k can defeat a 130k.
Different stats are weighed differently. One might have massive speed while the other has massive resistance. If you aren't doing any debuffs that can be resisted and are doing pure damage. The 50k will destroy the 130k.
That is why it says to take the Power levels with not as much attention. If your champions are build correctly and are strong champions in their own right then they will be fine.
The Teshada has a lot more of every single stat other than HP than the Demytha. The Teshada while the gear is lower upgrade level being only 12-14 it is also 5* or 6* gear. 5*+ gear has a lot higher base stats than 3* and 4* pieces like what is on Demytha. Teshada's set bonuses are also 40 Res and +10% Speed x3. This is an extra 120 Res. Which Res gives the most "power". While Demytha has on 6 Pieces of Regeneration. Which is a 4 piece set that gives +15% Healing a turn. If you are going for set bonuses 4 Piece Regen + 2 Piece Immortal would be better for +15% HP as well as +18% Healing a turn.
But you should be going for stats over sets as a priority. If a mismash of pieces gives you 4k Atk but you can use a set for +15% Atk but if that +15% ATK only moves you to 3.4k the mismash of 4k Atk would be much better.