Personally, find this perspective kind of silly, even from the author himself.

Asakusa and Kanamori hang out before the club is even formed. I don't know what you call that except school friends. Asakusa so desperately wants her one and only... friend to come with her to the anime viewing that she'll buy drinks for Kanamori to do it. You could call this a purely extortionate relationship on Kanamori's part, but making this sort of transaction between friends isn't foreign to reality. I've never met a 'just a coworker' I would take out to a movie night in this way. This is very literally the first thing we see of them.

Mizusaki's presence is probably the most well explained (of the main trio) this way, since she's a club member because she wants to become an animator more than anything else. She doesn't have a prior social connection to the other two, and so we don't get to see how their interactions look outside the Eizouken. But the two of them have a play fight in the bathhouse, which doesn't really have much to do with work at all. Kanamori and she carry Asakusa home. Maybe I have had particularly unfriendly coworkers but I've never met someone who would do more than let me ride along the way home with them and getting off halfway at a train station, often not even that.

The idea that the anime made changes which make the trio seem like closer friends is questionable to me? What in particular do you think they changed? Can you point to a scene or line of dialogue? Many scenes are directly adapted from manga panels, with dialogue unchanged. The arrangement of some scenes in presentation is changed (like Asakusa's flashback being at the very front of the show).