Skally, the words secular and sect are defined as follows: Sect:
- A group of people forming a distinct unit within a larger group by virtue of certain refinements or distinctions of belief or practice.
- A religious body, especially one that has separated from a larger denomination.
- A faction united by common interests or beliefs
Secular:
- Worldly rather than spiritual.
- Not specifically relating to religion or to a religious body: secular music.
- Relating to or advocating secularism.
- Not bound by monastic restrictions, especially not belonging to a religious order. Used of the clergy.
- Occurring or observed once in an age or century.
- Lasting from century to century.
n.
- A member of the secular clergy.
- A layperson.
The context of the article quoted at the outset seems to imply that the Watchtower is using the term "secular" according to defintions 1-3, rather than in the sense of the noun. Hence, Jehovah, the big Killer in the sky, pronounces lights out on religions and their supporters first (thanks to Blondie for her clarifying research) and then on the non-religious or "secular" part of mankind. That's my take on it anyway.
Good points also by Gumby and Dan.