I discovered that the write protection screw on acer 13 is labelled JP10, as mentioned earlier by DiskNoFun. In chromeos I checked the write protection status:
$ sudo flashrom --wp-status
The write protection was indeed disabled.
So I then tried to enable it with:
$ sudo flashrom --wp-enable
Rather to my suprise it was then enabled and I was unable to read from the spi. So I tried to disable write protection.
$ sudo flashrom --wp-disable
I was unable to do this so had to take the chromebook apart. I removed the JP10 screw and I was still unable to disable write protection. I had to scape of all the foil around the screw hole and after a couple of attempts was finally able to disable it!
So it seems that both reey's machine and mine were shipped without the write protection enabled. Possibly all the acer 13's were shipped like this under the Google radar. Theoretically the spi could have been flashed inadvertently.