the shirt is a little bit off, but mind palace john
is totally meant to be wearing the same thing as pool scene john
right down to the belt and shoes
this has to mean something, otherwise mind palace john would have just been wearing what actual john was actually wearing that morning
my money is on the idea that this is a representation of sherlock’s nostalgia/wistfulness/pining for a time when it was just the two of them against the rest of the world, at the very least.