I wouldn't describe anything with a JSON kitchen sink field as "normalized".
If it was normalized, there wouldn't be a problem. You'd just index the subscriber list and postgres would handle the = or in queries properly.
But json to postgres 9.3 is no better than an abitrary string that stores a delimited list.
This is a denormalized workaround for a denormalized schema, with big performance gains.
Article is very application-specific, but is perhaps a reminder that denormalisation can be useful when working with relational databases.