Container tabs help protect from cross site tracking : whether or not you subscribe. So that is irrelevant here.
What matters though is what they track, and what they do with it. The problem with most tracking is that there are services that aggregate it between sites such that your viewing habits in one are available for use in another. There is tracking in an effort to make your service better, including for who you're tracking, and there's tracking as a source of revenue. The first is not a problem, and not anything fundamentally different than what you could experience while using a corner store in 1900. The problem is it's very hard to confirm which is being done much of the time.
1. Information can never be un-leaked reliably. 2. Even if current owner of the tracking company behaves , you never know what the next owner in case of bankruptcy / strategic sale does with the data. 3. If some unscrupulous employee of the tracking company leaks your information, there may not be any proof about it. 4. You never know if the information becomes dangerous when combined with some other information which may be leaked / required to be given in some other context.
So credible guarantee is the only thing that can make paying with money, and paying with information a true dichotomy.
The simplest way for WaPo to achieve this without having to deal with any adbockers or trackers would be to use an in-house system served from their own servers. Unfortunately, since they also have a free subscription where they do track people, that would mean two systems which would be redundant, and the in-house one also probably takes a lot of effort and manpower. Unfortunately, that leaves subscribers not knowing exactly what is being tracked and who is doing the tracking.
I mitigate this by using ublock and a container tab for news sites in Firefox. I would prefer to know this isn't needed because WaPo did the right thing, but I would still do anyways, as just because something shouldn't be needed for protection doesn't mean you shouldn't do it anyways if the cost is small enough.