The report comes from website Axios, to whom Microsoft released the statement to saying they would not object to the union being recognized.
You can read the statement for yourself, here:
“Microsoft will not stand in the way if Activision Blizzard recognizes a union. Microsoft respects Activision Blizzard employees’ right to choose whether to be represented by a labour organization and we will honour those decisions.”
Though we don’t know much else regarding other issues like bargaining expectations and what conditions Microsoft may bring to the table, this is at least a step closer towards unionization.