I don't think the ARMS character will be delayed. I think they will indeed come this month. Looking back, the waiting time for Mewtwo in Smash 4 and Joker in FP1 was close to this one. I think the first character in the season might take a while longer to get out because they probably hadn't even begun production when they first teased ARMS - similar to what happened in the aforementioned situations.
And similarly, the delay between the rest of the DLC might be smaller, because considering their pipeline, they're probably working on more than one character at once.
If anything, this might be another indicator that a second DLC season wasn't planned at all when Ultimate launched.