I think this was the original plan because they had no idea how audiences would react to this. I think if this movie is as good as it's looking and does well they may change that. There's supposedly a bunch of easter eggs that if they decide to take them as more than easter eggs could tie this into the greater DC universe. But also I think the way the time frame works on this is that it'd definitely be possible for a 10 year older version of this joker to meet a early career Batman. DC / WB have been all over the place, so I wouldn't put it past them to change their minds on this not being part of the main "cannon"