Very uninformed speculation. Visionect's previous products came with a water-tight, tamper-proof enclosure & a number of software based security measures, because one of their tablets interfaced with Point of Sale systems.

This is a bit more complex than a regular micro-controlled LCD. There are a number of ways you can drive an e-ink display. Then there are a number of ways you can choose to control such a display remotely. Last but not least, a sizeable chunk of the stack powering these displays is proprietary technology, and has been in development for more than 3 years.

Almost everything is hackable when you have physical access to the system. That's why such systems should detect and report tampering. Something regular traffic signs never did in the first place.