The player ID for the octolings was a shortcut for their functions in single player mode. This has been said repeatedly on Drlilrobot's twitter, which Nikki now also seems to believe. I am no coder or game designer, but it may have bugged the creators as well to have the octolings stuck with such crap models. Any artist would like to complete their work.
Not all of the update work has been on the octolings, in fact, most updates have been on the servers, weapon matchups, stages, and of course adding gear and the like. If they were intended to become playable, I wouldn't put it past Nintendo to suddenly scrap it all. I wouldn't put it past any gaming company, really. If they can come up with a reason for the inclusion, for continuity's sake they'd cut it out.
Do they function like inklings when in multiplayer mode? Yes. Does this mean it's solid evidence that it will happen? No. They are an offshoot of the inkling girl model--it's not hard to see that they have the same exact movements and faces, given they have slight alterations and difference in species.
I am not anti-octo, nor am I entirely pro-octo either. I'm neutral on this stance, thinking logically instead of being blinded by fan rage. I would love more than anything to play as an octoling-- I would PAY for the DLC, but there's just no clear indication that it's happening. There are codes that seem like indicators such as the player ID, but that simply doesn't make it positively certain that we'll get them before a sequel hits.
.........And don't forget, Nintendo is the biggest tease ever with their updates, vague news, and tweaks to their sites--they could just be hyping people up for nothing again. They've done it before.