It's difficult for me to get a clear answer to whether content-locking is ethical in general, so I'm just going to focus on Splatoon right now. One of the ways this makes things easier is that I can list the locked content:
1) Amiibo-locked content
2) Unreleased maps and weapons
3) Map rotation
4) Unreleased game modes
More in depth:
1)
I don't know if Amiibo are quite relevant to what the article is talking about, as they're locked behind extra payment rather than time, but as it is still locked content, I find it useful to discuss.
Essentially, the Amiibo unlocks seem to be equivalent to on-disk DLC. In principle, I don't think this is a very good idea, but in this case, the unlockable content is so minor as to be insignificant to those who don't have Amiibo. The extra costumes are merely cosmetic, but the new ways to play story levels and the new minigames, while not very important, seem like the sort of thing that adds to the game in a non-trivial way. Essentially, these things can enrich the game, were developed beforehand, but are not allowed without spending extra money.
As such, while I think that hiding some of this content is of shaky ethics, the matter is so minor that I don't really care. However, I do have all the Amiibo, and do enjoy their benefits, so I would like to hear from someone who does not.
2)
As for this unreleased content, I think it's perfectly fine. With the large variety of weapons that the game starts with, withholding some isn't a problem. If the game started with only, say, shooter-type weapons, and we had to wait for chargers and rollers, I might be displeased, but we start with all kinds.
As for maps, I have essentially the same thoughts as the weapons. They've given us a nice variety, and very quickly have released more.
The main problem that the article seemed to have with this release strategy was its use as an incentive for people to play the game in shorter bursts, rather than play it all at once. I think the article is probably right about why the content is released why it is, but I disagree that this is a bad thing. The reason I disagree is, as I stated, we started with a full complement of weapons, and at least a good selection of maps. I don't mind Nintendo trying to get me to play the game over a longer period of time, as long as they don't do it by gradually lifting frustrating limitations, like the free-to-play games the article mentioned.
3)
As for map rotation, I don't think the purpose or effect of this is to cover up the number of maps, but rather to encourage people to think about what weapon set will work best for the pair of maps presented. I'm not sure if like this style of choosing maps better than a more traditional one, but I think this issue is more related to "what is good game design" than locking content.
4)
This relates to both the addition of new game modes (Splat Zones, Tower Control, etc.) and the addition of ranked matches. This is the most ethically dubious area to me. I realize this changed within a few days, but the game started with one multiplayer game mode, unranked Turf Wars, which I feel is actually a problem when, at the time of release, they had several other finished game modes. I realize what Nintendo is trying to do, letting people get used to one game mode before offering another, but this seems like something that consumers should be able to decide for themselves, and as it relates to a significant, non-trivial gameplay addition when the original offerings are sparse, it doesn't seem right to withhold it like this. Of course, the initial additions here were very quick.
Concluding stuff:
Some of Splatoon's content withholding seems to be somewhat problematic from an ethical standpoint. However, most of it is fine. I, for my own part, immensely enjoy the game, and have done so since launch, and I don't mind, and even in some cases appreciate how the content is rolling out. I think that the content withholding should be judged by the form that the game takes without extra content compared to the game's form with additional content, and from this aspect, the game does very well even from its start, and I don't think Splatoon significantly changes with the extra content, though it is certainly appreciated.