It's interesting to note that he explicitly mentions "November 1966" and also refers to Simula. The ACM paper about Simula I appeared in September 1966 (see https://dl.acm.org/doi/10.1145/365813.365819), so he might have read it. Simula (or Simula I as it is called today) was a language to support discrete event simulation and had active objects (called "processes") for that purpose which "sent events" to each other (i.e. the process is activated at the event time). The language was already presented in 1962 at the second international conference on information processing held in Munich. The actually "object-oriented" Simula version with classes, inheritance and virtual methods appeared in 1967. Kay's vision applies to Smalltalk-72 to some degree, but Smalltalk-76 then also switched to inheritance and compiled virtual methods.