If 2 drs, say glaucoma was caught early but there is 30% optic nerve damage and some peripheral vision loss, how can they say it was caught early?

Damage v. symptoms: Glaucoma may have many variable presentations and, depending on the location of optic disc damage, may have focal notch to diffuse damage, both differing in field loss measurement. A 30% disc damage isn't necessarily equated to 30% field loss. In fact a 50% disc damage may have a normal field. So it depends on the extent & location of the damage & wether in breaks through the neural retinal rim.

Glaucoma: Our optic nerves have a lot of redundancy built into them. It takes more than 50% loss of axons in the optic nerve for vision loss from glaucoma to become noticeable.