I may be wrong, but I believe the term "frozen ATPL" arose when the CAA were NOT accepting CPL theory + IR theory as sufficient theoretical training for grant of an ATPL i.e. you had to do the specific ATPL theory exams, and by saying "frozen ATPL" indicated that you had completed them.
Seems to me the difference between CPL+IR and ATPL is purely your level of flying experience -- not a measure of qualifications
IMHO there is a subtle difference beyond flying experience. The skills test for grant of an ATPL requires "the applicant shall demonstrate the ability to perform as Pilot-in-Command of a helicopter type certificated for a minimum of two pilots
under IFR"...
An ATPL shows demonstrated competence operating within a multi-crew environment. A CPL shows demonstrated competence operating within a single-pilot environment.
Suffice to say, flying as PIC in a multi-crew environment does not mean you fly it like it was single-crew!