By definition full NANDroid backup means exact image of the phones NAND storage.
TWRP backup never included emulated Internal Storage and is technically no full NANDroid backup. The short answer is NO.
Long answer
TWRP main purpose is backup of ROM, apps and settings.
Backup of firmware depends on how the TWRP maintainer predefined selection of partitions for this phone model. Having a look into specific twrp.fstab the file only has 28 lines.
Providing the average user the power to backup and wipe all the partitions can be considered as dangerous. Furthermore we can download firmware elsewhere, backup of firmware via the TWRP backup menu is therefore not desired. That's why some maintainer's exclude firmware partitions from menu.
Advanced user can backup all partitions manually from adb pull
as TWRP provides full access to raw partitions.
Backup of ROM is still functional but the backup method has changed. the system partition is no longer a tarball archive containing files, but a partition image.
TWRP can backup dynamic partitions inside super partition since v3.7.0
Backup of Data requires the userdata partition decrypted. Backup does not include /data/media aka /sdcard Internal Storage. Therefore one must backup pictures manually via USB MTP or adb.
adb pull -a /sdcard
Android Keystore and apps that rely on security cannot backed up due the nature of Replay Protected Memory (RPMB)
This concerns some crypto or banking apps, screen lock credentials and FBE file-based encryption.
Adoptable Storage encryption rely on phone encryption. encryption keys cannot backed up and therefore all data on MicroSD card becomes unrecoverable. manually backup of decrypted files is possible.
island/work profiles/parallel apps cannot backed up because TWRP cannot distinguish between encryption profiles, only user 0 is decrypted by default.