Here's a bit more about these people.
and
They could work with copper, produce a wide array of ceramics, and they lived in well-planned dwellings and towns. And apparently they invented the wheel before anyone else, which is kind of a big deal. It's not at all far-fetched that they might develop some semblance of a written language.
A lot of their symbols seem to be simplified representations of more complex forms, which is only to be expected of a writing system. Like Roman numerals, or even on a modern keyboard.
Symbols drawn with just one straight line:
I / - \
Two lines:
V X = > ^ T L +
Three lines:
4 A F H K ] [ N Z Y
etc.