The greenhouse emissions problem depends on where that garbage would otherwise end up, and what would happen with it.

If the garbage would otherwise be recycled fully then incineration would seem like a worse option. But if the garbage would otherwise end up in a landfill, it leads to decomposition and methane production. Methane is one of the worst greenhouse gasses out there, so incineration might be preferable to just leaving it be.

I’m no expert whatsoever, so take my comment with a grain of salt… I may be misinformed