Yeah that was the rumour. No formal offer was made otherwise they would have had to put it in the SEC filing. Instead the
SEC filing states
"In addition, the Activision Blizzard Board of Directors and senior management from time to time have been approached by third parties expressing an interest in exploring a potential strategic combination with or acquisition of Activision Blizzard, but, since the reduction of Vivendi’s ownership stake in Activision Blizzard in 2013, no such discussions have advanced beyond preliminary discussions gauging each party’s respective interest in pursuing a transaction
or resulted in any specific proposal on price, structure or other material terms."
" On November 19, 2021, in the course of a conversation on a different topic between Mr. Spencer and Mr. Kotick,
Mr. Spencer raised that Microsoft was interested in discussing strategic opportunities between Activision Blizzard and Microsoft"
So it seems if the rumour is true that Activision was given a proposal by MS. Did his due diligence of seeking to see interest by other companies, got no formal offers of interest (otherwise it would have legally been in the SEC filing) then negotiated with MS on the price.