“Leanest” is, imo, the perfect word in the English version, not only because it can easily (and unambiguously) be used (as it is here) in its non-pejorative “fit and trim” (en pleine forme) sense (“It’s [s/he’s] a lean[, mean] flying [fighting] machine”), but also because one can “be” lean [and therefore be built lean from the ground up] without the automatic assumption that one has “gotten” or “become” lean (which makes it fit well with the claim that the C Series wasn’t created by tweaking [making leaner] an earlier [and less lean] design).

On the other hand (and again, in my opinion), the French version’s use of “plus allégé/s” falls well short of being “le mot juste” (my apologies for, except for the preceding three words, writing this answer in English).

Granted, regarding the “non-pejorative” nature of “leanest” in this context, “allégé” is certainly no more likely than “lean” of being construed negatively when discussing an airplane or its design (as opposed, however, to other, more standard translations of “lean,” such as “maigre” or “mince,” which might not necessarily be seen as positive descriptions of a commercial aircraft).

As it applies to the claim in French that this plane wasn’t [just] created “en modifiant ce qui existait déjà : ils ont été spécialement conçus à partir de zero,” however, it seems that, unlike “lean/est,” the word “allégé” (as I interpret the entry you cite for it), necessarily requires, in contradiction to Bombardier’s claim, a pre-existing object/design that has [“simply”] been tweaked/modified to reduce its size and/or weight.

Of course, the use of the superlative (with either “leanest” or “le plus allégé”) generally implies that something else (of lesser leanness/?allégéance?) is already in existence, but I don’t think “the leanest” implies as strongly as does “le plus allégé” that some pre-existing thing less lean/?allégéant? was necessarily tweaked to become “the leanest.”

Regarding what “un avion le plus allégé” might mean (and what some other adjectives might be), the English notion of “bare[-]bones” first came to mind, but describing a plane with that term would be no less potentially fear-inspiring to its English-speaking passengers than would describing it as “mince” or “maigre” be to its French-speaking ones.

However, the notions in some of the adjectival translations of “bare bones” given in the above Reverso link (i.e., “basique”, “épuré”, “réduit au strict minimum” … “réduit à la portion congrue” ) led me to the apparently archaic/rarely-used-except-in-fixed-expressions meaning of “congru/e” (“Qui convient exactement”), and from there, to its top-listed (per CNRTL) synonym:

“convenable”, to which I’d add the notion of “au but” =

L’avion le plus convenable au but/à son but/à la tâche.