CX seems to be taking the position that as the dispute is not between them and their staff directly, they view taking part in a general strike as illegal. The question becomes “is this an action sanctioned by the various CX staff unions?”, as the EO allows for union sanctioned activities and makes no requirement of a dispute between the employer/employee.
The HKAOA has made their position on this strike clear however.
Article 27
Hong Kong residents shall have freedom of speech, of the press and of publication; freedom of association, of assembly, of procession and of demonstration; and the right and freedom to form and join trade unions, and to strike.
The HK Trade Union's Ordnance defines a strike as “the cessation of work… in consequence of a dispute, done as a means of compelling their employer… to accept or not to accept terms or conditions of or affecting employment.” Although Article 27 gives a person the right to strike, the ordnance says it's only in the case in the case of a grievance against one's employer regarding working conditions. In this case, the company are correct. Anyone participating in a general strike will technically be absent from work.