MrAverage, the rule in FCL.915(b)(4) requires that "... holders of an instructor certificate with privileges to conduct flight instruction in an aircraft shall... be entitled to act as PIC in the aircraft during such flight instruction." The rule does not specify the basis on which the instructor shall be so entitled. Acting, or being entitled to act, as PIC under the privileges of a non–Part-FCL licence is sufficient. The same goes for examiner certificates.
One example is addressed in point (c) of FCL.900—Instruction provided outside the territory of the Member States [or the UK]. For example a person holding a UK Part-FCL licence (CPL or higher) may conduct EASA flight instruction at a UK-based ATO, which is approved by EASA, without being the holder of a valid EASA Part-FCL licence.