That’s a really good question. My interpretation is that the termination notice exemption applies only to the field and not the office workers. Reason: Section 5(1) of the ESC Regulations, which I’ve pasted in below. Note the boldface language (the boldface comes from me, not the actual Reg.)
No notice of termination

5(1)  No termination notice is required to be given by an employee, and no termination notice, termination pay or combination of termination notice and termination pay is required to be given or paid by an employer to terminate the employment of an employee if

(a)    the employee is employed at the site of and in the construction, erection, repair, remodelling, alteration, painting, interior decoration or demolition of any: etc. etc. etc.
Employees of an offsite office wouldn’t meet that definition. Hope this helps. Glenn