I would say yes, but I am a legal layman. Some legal experts say that forced labour is a broader definition that includes but goes beyond slavery.
The International Labour Organization (ILO) defines forced labour as
all work or service which is exacted from any person under the threat of a penalty and for which the person has not offered himself or herself voluntarily.
The NGO Anti-Slavery International comes to a similar definition;
Forced labour is any work or service that people are forced to do against their will, usually under threat of punishment.
However we define (and call it), it's a devastating situation for the people, in that case, North Koreans. We have similar things in China (particularly in the Xinjiang Region), and many other countries.