Pretty sure this is fungal leaf spotting caused by lawn sprinkler overspray but thought I'd ask to be sure. This happens mainly on the oldest and lowest fronds.
I have the same spots on my sabal birmingham. Im not sure what they are either but it definitely looks like some type of fungus. Mine has it on the older fronds and was there when i bought it so Im not sure. I did some research and it looks like leaf spots/blights.