Sorry I think you might just be mistaken. If your greenhouse is reaching 150c then there's actually not that many common plastics that wouldn't hit the glass transition temp. There are some exotic "plastic blends" like glass or carbon fibre/mineral blends that can do it, but even most of those are less than 140c. In this table even the modified versions of polypropylene have glass transition temperatures below negative: https://omnexus.specialchem.com/polymer-properties/propertie...
I don't think it should be too surprising that 3D printing filaments can withstand similar characteristics to other outdoor plastics. One commonly used outdoor plastic is PVC which has a glass transition temp between PETG and ABS: PETG: 80c, PVC: 85c, ABS: 103c. If you left a PVC pipe in your greenhouse you wouldn't expect it to deform.
I don't think it should be too surprising that 3D printing filaments can withstand similar characteristics to other outdoor plastics. One commonly used outdoor plastic is PVC which has a glass transition temp between PETG and ABS: PETG: 80c, PVC: 85c, ABS: 103c. If you left a PVC pipe in your greenhouse you wouldn't expect it to deform.