I have had a further look around at flashrom, glibc and apu2.
The message that flashrom requires glibc-2.34 doesn’t make sense as the version of flashrom running in IPFire is 1.2 and was updated in IPFire in January 2021.
flashrom-1.2 was released in February 2020.
glibc-2.34 was released in August 2021 so around 18 months after flashrom-1.2 was released so flashrom cannot be the program that is demanding glibc-2.34
I suspect that apu2_v4.16.0.1 is the program that has decided that glibc-2.34 is the minimum required.
In none of the changelogs from flashrom or apu2 is there any mention of an increased run dependency on glibc to version 2.34
Currently IPFire is on apu2 version 4.15.0.1 so any upgrade will occur with CU167 or later so glibc will be at or above version 2.34 at that time.
Conclusion is still that an update to CU165 should allow you to do the firmware update successfully.