This is the legislaton? https://www.elegislation.gov.hk/hk/cap494A

4. Prohibition of entry, etc., of restricted area without permit
Subject to this Regulation, no person shall enter or remain in a restricted area unless he has on his person a valid permit issued to him in respect of that restricted area or is being escorted by a permit holder who is authorized by the issuing authority to escort persons into such area.
5. Exemption for bona fide passengers and air-crews
(1) Subject to this section, the provisions of section 4 shall not apply to—
(a) any bona fide air-crew member or bona fide air passenger who is in the course of passage through any immigration control or aircraft boarding area or an area set aside for the purposes of the Customs and Excise Service, which is within a restricted area—
(i) having disembarked from an aircraft; or
(ii) for the purpose of embarking upon an aircraft of which he is an air-crew member or air passenger;
(b) any bona fide air passenger awaiting an onward flight in any area reserved for transit or transfer passengers which is within a restricted area.

Alright, so OP was exempted from 4.'s "no person shall enter or remain in a restricted area" because they were an 5.1.B "bona fide air passenger awaiting an onward flight" (they did not want to not take the flight). OP then didn't want to continue waiting airside, so then 5.2.B is not longer true, so it jumps back to 4's "no person shall enter or remain in a restricted area". So OP followed the law to the dot, and asked to not remain/and be let out.
Note 5.1.B said "awaiting", whilst OP was drinking champagne in CX Flounge, were they not waiting for their flight?

Meanwhile, let's look at 5.1.A "bona fide air passenger who is in the course of passage" and 5.1.A.2 " for the purpose of embarking upon an aircraft".
Therefore, for those FTers who are throwing the book at OP, please be informed that when ex-HKIA, thou shall head directly to the aircraft, with no deviations to the lounge.

Thank you.