It's an attempt to normalise they/them as valid pronouns and thus normalise gender and pronoun ideology. We are witnessing the indoctrination of very young children and their families.
The explanation that 'it's just an alien so has no gender' is disingenious of course. The fact that this genderless alien is a they/them makes it all about the T. The they/them character is going to be pictured on kids' library cards and be used for branding.
Libraries are all about trans now. We no longer go, I'd never allow my older dc to pop into the library alone (my best childhood memories were browsing my local library by myself) but we can luckily afford to buy books, so that is what we do. People also might chose to have ad hoc communal 'libraries' and book swap schemes with local friends.
I hope Hertfordshire council will reconsider this move but won't hold my breath.