IMO the likeliest explanation is the lethality of novichok is way overblown or generally misunderstood. Skripals' poisoning had a very similar progression (the two targets survived after a coma [0], another inadvertently exposed person survived, one has died). Most anecdotes about Novichok's incredible lethality are stories out of Russia where use of Novichok was merely alleged. It's weird they'd botch 2 high profile assassination attempts like this if upping the dose a bit guarantees death.

[0] I'm not too familiar with the exact mechanism that induces the coma. But common sense would tell me extended comas after which the patient makes a full recovery are an unusual phenomenon medically. Navalny's doctors seemed pretty unsure about his prospects too. Yet both Skripals and Navalny not only escaped death but had full recoveries from extended comas.