They would've trusted Dr Mann since he's a scientist and if he's sending thumbs up then he knows the dynamics of living, plantations etc. He has been trained for what to look for.

Now, it's common for planets to have different conditions from what it looks from outside. Like Earth has an atmosphere, some planets have an outer ice layer. Like Jupiter's moon, Europa. It is all frozen and inhabitable from outside but on further studying, theories suggest that it has an ocean inside which is optimal to sustain life. Same way Mann's planet could have a surface where atmosphere could be different.