Il me semble qu’ici on pourrait remplacer au fond par en fait sans pervertir le sens de la phrase.
Au fond à le sens de “à considérer le fond des choses” (et non l’apparence, la surface), (Cf. En réalité) — le petit Robert.

En réalité, le temps d’une année a semblé très cours au narrateur.

Les deux traductions sont des transpositions intéressantes, car elles développent toutes les deux la rapidité avec laquelle le temps est passé entre deux étés.

Au fond insiste, pointe, met en valeur cette rapidité, c’est ce qui est après coup le plus important.

Stuart Gilbert développe complètement ce qui est induit par la phrase d’Albert Camus.

Matthew Ward garde la concision du texte français, en transposant au fond en honnêtement comme marqueur du temps écoulé rapidement.

N’étant pas anglophone, at bottom semble convenir, mais dans l’exemple proposé il s’agit d’apprécier la valeur d’un homme, pas de valorisé l’écoulement du temps.
Ce devrait être une traduction possible, compréhensible, très mot à mot.

Les traductions citées indiquent que la transposition retenue reflète l’atmosphère du texte original.

au fond means at bottom, but it also means honestly depending on context.

The context of the sentence changes the meaning, but au fond can mean honestly, although at bottom is better

Two professional translations didn’t rely on the at bottom idiom; that must mean something (beyond a two-liner statement to the contrary). Whether it’s because of the declining use or because it’s chiefly from the BrE variety, or UK formal even, and they were interested with a more personal register, in any case they didn’t use it. At bottom may mean any number of things (in reality; basically or despite appearances to the contrary / Collins; basically / AHDotEL; essentially / MW; Dictionary.com, in reality, fundamentally, links the idiom to at the bottom of, as in the ultimate cause of). The function is that of a(n) (phrasal) adverb. But even if one equates at bottom with au fond, that’s not what translation is about.

Now insofar as the French language is concerned, au fond/dans le fond means « Tout bien considéré, en dernière analyse. » (TLFi) or « en réalité, tout bien considéré » (Larousse) i.e. wfw : all-well-considered/in-last-analysis/in-reality. It is independent from the ground up or essential propety/characteristic meaning such as with le fond des choses, alluded to elsewhere, and is rather related to a figurative use of a secret, or intimate location. It is also functionally a charnière (hinge) construct used to summarize, so this is not only about semantics. Constructs with a similar function include: « À bien considérer les choses, Ainsi, Au fond, Au total, Bref, Dans l’ensemble, En bref, En définitive, En résumé, En somme, En substance, En un mot, Essentiellement, Mais réflexion faite, Pour tout dire, Somme toute, Tout bien considéré, Tout compte fait. » (BDL). One meaning for honestly is really, genuinely (MW) which is certainly not inconsistent with those. One could say one went with an expression for a summary whereas the other chose the genuine, straight up statement to the same effect. On the whole is basically somme toute; I personally wonder why Gilbert didn’t do on the whole I must say another summer was on its way before I realized the first was over, but I don’t have sufficient (native) proficiency with the language and haven’t been trained to translate. In any case that’s not directly on topic, and I would think consistency and being faithful in terms of register require looking beyond a linear word equivalence, or one sentence for that matter, as I’m sure you know. I’m not familiar enough with the work to tell, but generally here someone is coming to terms with how the year went by (fast, despite whatever), using summer as a point of reference. Both translations work imho.

Au fond, is not at bottom. It means: substantively. As regards the substance of something. With regard to its essence or substance. It is used in French law, les questions quant au fond: the issues as regards the substance of a matter. Fond is opposed to forme in French. le fond et la forme de quelque chose. The substance and form of something. As an adverb, it means: fundamentally, essentially, basically, as to substance. Sorry, no dictionaries as I am sharing what I know.