On the contrary - that article seems to show that the London fix is a free market process. If an individual bank wants to increase the price of gold they must commit to buying gold at great volumes to cause that market movement. OTOH if all the banks conspire to move gold in some direction, any one of them can betray and profit at the expense of all the others knowing the movement is artificial.

You can’t fix prices unless you’re either willing to sell enough or buy enough to fix it. If you can’t do that then eventually the peg breaks. See the collapse of the dollar gold peg in the 1970s when France wanted gold from the US, or the collapse of the UK’s peg to the DM on Black Monday.