There were still around 50,000 people who remained in Phnom Penh or were sent their after the liberation in April '75. There was very small scale manufacturing and textile work, as well as automotive repair. There were diplomatic stations from a handful of friendly nations - even a shop they could get things from. Cambodia still had an airport and electricity in Phnom Penh. They had a large military and they certainly patrolled the borders look up the 'ss mayaguez incident' where a merchant boat was captured by the KR, they also entered Vietnamese territory on several occasions which led to the war between the two nations. They did not close the country off completely, China was their primary benefactor whom they traded with via air and sea. Technologically they did rely heavily on 'man power' to complete certain projects which would have been easier using more modern agricultural techniques or industrial machines. But they didn't pretend it was a different century, the idea was to produce extreme surplus through agriculture that they could modernize an industrial sector in a very short amount of time.