The biggest issue is that it's a very simple model that runs very quickly, which means it's unlikely to capture any dynamics that rely on any kind of spatial heterogeneity, ocean circulation, and so forth. There's likely feedback mechanisms that simply aren't captured in their model, and it's not clear whether the feedback mechanisms it does capture are accurately modeled. The paper acknowledges these shortcomings and suggests that more complex models should be run to verify their results. There's good reason to be skeptical of this particular result until that happens.
The biggest issue is that it's a very simple model that runs very quickly, which means it's unlikely to capture any dynamics that rely on any kind of spatial heterogeneity, ocean circulation, and so forth. There's likely feedback mechanisms that simply aren't captured in their model, and it's not clear whether the feedback mechanisms it does capture are accurately modeled. The paper acknowledges these shortcomings and suggests that more complex models should be run to verify their results. There's good reason to be skeptical of this particular result until that happens.