Yeah, it's flexy all right. I have three Litespeeds, a 1996 55cm Catalyst, a 2006 57 cm Tuscany and a 2006 57 cm Firenza. Two of them have the same BB6500 Octalink bb and Ultegra cranks you have and one (the Tuscany) has a Campy Chorus square taper bb and crank. None of them have anything close to the deflection your Merlin shows in the video even with my full weight on an 8 mm hex bit in a full length 3/8"-drive ratchet.

All I assume is that the Merlin must be an Ultra Light and those carbon seatstays contribute very little to the stiffness. Is it a big frame, say 60 cm or more? As noted, a different bottom bracket and crank aren't going to improve the situation. This bike is an excuse to learn to use a fast cadence in lower gears.