At a quick skim, I'm not sure this contradicts what I'm saying. Yes, large trees that are getting larger are excellent carbon sinks. On the other hand, there aren't many such large trees, and they tend to outcompete other growth.

From the article: Second, our findings are similarly compatible with the well-known age-related decline in productivity at the scale of even-aged forest stands. ... We highlight the fact that increasing individual tree growth rate does not automatically result in increasing stand productivity because tree mortality can drive orders-of-magnitude reductions in population density. That is, even though the large trees in older, even-aged stands may be growing more rapidly, such stands have fewer trees. Tree population dynamics, especially mortality, can thus be a significant contributor to declining productivity at the scale of the forest stand.

My point is contained within "the well-known age-related decline in productivity at the scale of even-aged forest stands" that they refer to. That said, I agree that this point is not universally true and depends on a lot of local factors.

As the average tree in a forest keeps getting older you keep sequestering carbon and older trees are sequestering faster than young ones. The issue is eventually you reach an equilibrium point where the average age becomes a constant because trees have a finite lifespan.

Globally most forests were cut down relatively decently so they haven’t reached equilibrium. But when you start talking the next 100+ years it does become an issue.