True. Weight alone isn't everything, but every additional obstacle you can put in front of the crims helps.
A single burglar obviously isn't going anywhere with a 150kg safe by himself. I dunno about 150kg being a "2-man-lift" any day of the week... 2 guys with a trolley though sure.
Obviously if they come prepared to remove a safe then that's one thing, but for the most part... If you don't make a spectacle of the fact that you're a shooter though I think it's fair to say 99.9% of random burglars won't be in a position to get away with a 150kg safe at the time.
Compared to your average 40kg 'lock box' style safe which literally anyone can carry out with little effort if they manage to unfasten it, 150kg is still a huge plus just for the 'awkward factor'
I've got a 150kg + model and it took me and 3 mates to get it where it is just because of limited space and crappy angles required to get it in there.
Mine's bolted in as well anyway, plus they have to deal with getting over the fence, getting into the house, the alarm, dogs, two pugilist neighbours who work from home and I live in a place with regular foot traffic (see: witnesses) around the neighbourhood. Not worried about my safe

Every layer helps, and I reckon the weight is a particularly useful one. My 2c anyway.
To OP, if you really wanted to plant it in the ground you could spend an extra (I think) $250 and get the 270kg SFB1.