well we do see that there are some proof of that given the clock symbol on the corners of the boxes and how the giant snake mech have that symbol on it too.
Plus we have the part that Nya and Kai's parents does not look old at all and instead look just like they did during the first Serpentine war.
again, its possible that the story of the 2017 Ninjago sets could be because the Ninja travels back in time, and we also have these new villains, that might not actualy be part of the same timeline but instead a group of villains that want to change the past so that the Serpentine wins the war, it would explain why we have these new Snake villains.
plus the main object that everyone fights for this time seems to be some odd looking arm bands, that the villains want to use for their mech.
again, Time is clearly the main point here. And given how, again, Nya and Kai's parents does not look old at all, it just seems clear that our heroes will be going back in time.