If the most recent news story for the franchise is speculation on a Switch port, then wouldn’t an assumption of a tease be reasonable? Otherwise, what’s the point of this besides trolling? Someone would be exhibiting pretty poor discretion timing-wise for a tweet of this nature.
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A pretty common mistake is assuming that all of the people working at a company (4k+ at Blizzard) are as engaged as the players are with ingesting every scrap of rumor and news at all times. :)
What WOULD be a little more careless is if it is actually in development and then they put out this tweet, not thinking it might be misconstrued.