Honestly, I kind of get where Labor's coming from -- and I'm saying that as a Greens voter. Obviously, Labor's in the awkward position of being the most central of the three most viable parties, so they're vulnerable on both their flanks. Go too far right and their inner city seats go to the Greens, go too far left and they lose any affluent suburbs to the Libs (and the regions to the grab bag of mouth breathers from the Nationals/One Nation/PUP.) Traditionally Labor-leaning blue collar workers seem to be drifting towards the mouth-breathers recently, so there's definitely a clear need to fight for any votes they can.
The other issue is, they're always competing with a hostile media landscape -- Murdoch's the lynchpin of that, but also Costello's Fairfax, Kerry Stokes, and sadly even the ABC as of late. It's absurd but, there's this consistent straw-boogeyman of, ahem, "the chaos of the Rudd/Gillard/Rudd years, the leftist Labor/Greens alliance, rah rah rah" that will get wheeled out the instant Labor and the Greens seem like they're getting along. It worked during the Rudd/Gillard/Rudd years and it's not like the right-wing has any imagination, they'll just spew up whatever worked before, decades after the fact (Rudd/Shorten/Albo is the most left wing Labor leader since Gough Whitlam, oooOOOooo)...and tbh, it'll probably work.

At the end of the day, if Labor and the Greens were to put all their efforts into different electorates so they don't cannibalize each other's seats, act in, I dunno, some sort of coalition, the Coalition would scream bloody murder about how two parties colluding and working together is terrible for democracy, how the hopes and dreams of everyday Australians will be smothered by this unrepresentative group of career politicians, how the Red/Green menace will take all your money, destroy freedom of thought, and institute some sort of impoverished dystopia where we'll be made homeless and have to rely on food banks to survive. Yeah, I know, that could never happen here, but the media would signal boost that narrative and Labor's soft right flank would abandon ship.

IMO I reckon this silly 'a vote for the Greens is a vote for the Liberals' campaign is probably 50% a cynical attempt to grab the 'heart in the right place but dumb as a sack of hammers' Greens voters, and 50% product differentiation, that makes it that little bit harder to pretend Bandt and Albo are about to touch tips. If it came down to a hung parliament, of course Labor and the Greens would work together, but Labor cannot actually say that out loud if they want to flip enough seats from the Liberals.