I think you underestimate how rigid metals are. Say I wrap my battery in a 10mm steel box, similar to what EVE seem to be doing when welding. Having 250mm height and 130mm width I get 2 x 250 mm x 10 mm + 2 x130mm x 10 mm = 5000mm² + 2600mm² = 7600 mm² cross-section area.

Or, let's use aluminium instead of steel, since it's not even half as rigid at 69 GPa.

If we apply 50kN over 7600mm², that gives 6.579 MPa. That creates a stress of 6.679 MPa / 69 GPa = 0.0000967. Over the length of 8 of my 36.35mm cells, a total of 290.8 mm, that gives 0.0000967 * 290.8 mm = 0.02mm of elongation if I'm not messing up my calculations. Or, 0.0035mm per cell.

Using 6 M16 rods of 200GPa steel we get 6 * 125mm²= 750mm². Applying 50 kN we get 66.67 MPa. Stress = 66.67MPa / 200 GPa = .00033. Over 290.8 mm we get 0.097mm elongation. Or 0.012 mm per cell. We can continue calculating how much this would decrease the pressure, but I don't think it would be that much..