The first is actually not a mistake, but rather a creative writing technique to metaphorically relate one thing to another thing. A similar example:
Her speech was pure John F. Kennedy but with an Obama flair.
Obviously a "speech" can't be a person, so consider it shorthand for "like Kennedy's speeches". In a similar way, your example isn't saying that the columns are a memoir, but rather they include memoir-like elements.
Since this is a kind of literary device, I wouldn't recommend using it until you get a good feel for the nuance. And, as with any creative writing technique, it's best used sparingly, for special emphasis.
You might also see something similar, but with the adjectival "-y" or "-esque" suffixes appended to the noun.
The speech was Kennedy-esque, but with an Obama-y flair.
However, these weaken the comparison. The simile "A is like B" is less definite than the (literary) metaphor "A is B". (more on simile vs. metaphor)
As you might expect, the writer's creative use of "a stocktaking" is also not incorrect. Still, as it plays games with the grammatical structure of the sentence, it's a little disconcerting even for native speakers. The core sentence is:
The Atlantic columns are a stocktaking (of a certain series of events).
The countable "stocktaking" is a kind of shorthand for "a means to take stock", which is to say, "to list or record in detail". Another example:
The acrimonious board meeting turned into a pointless stocktaking of the current CEO's faults, rather than a discussion of how to fix the company's current problems.
Even though your sources say "stocktaking" is uncountable, I don't see anything wrong with this use.