On reassembly, I realise why it was so hard to get out—I had not lifted the retaining flap, thinking they just meant the flap of tape. Not well illustrated in the photo. The retaining flap is the long bit of plastic opposite the ribbon (closer to the fans).
Don’t forget to turn the logic board over (as pictured) first!
I agree, this step was unclear. Part of the problem may be variations between logic boards. Mine did not look like the one pictured here. Mine has a tab lying flat against the logic board, as also described by xtophr. If this describes yours, check out [|this video] https://m.youtube.com/watch?v=-IE3WNOsrZ... (around 40 seconds in) to see exactly how it comes out. You lift the locking clip up by the plastic tab, then (with a little bit of downward pressure on the cable) slide it out towards the nearest corner of the machine.
I had trouble getting the cable out (let alone back in!) In the end, sliding a toothpick in behind the ribbon and gently pulling from both sides of the toothpick did the trick.