It's being used in the singular sense.

I get it, but I thought it's merited in cases where the gender of the person referred to cannot be determined - usually because the reference points to an abstract class of people of any gender ("a manager") and not to a particular person of known gender ("John Smith"). In OP's case, the gender of their significant other is known, so why use "they" instead of "he" or "she"?

Everyone's assuming modern notions of 'identification', what happened to good old anonymity? Perhaps 'ReactiveJelly' just doesn't want to reveal (or rather probabilistically point at) their own gender, or that of their 'SO'. It's not like they said 'wife' or 'husband' and then 'them'.

It seems appropriate to use whenever their gender is irrelevant.

In this case, it creates extra confusion and extra mental overhead, because of double meaning of "they".

I understood what they meant perfectly well. Are you perhaps not a native English speaker?

do you also find the indeterminate number of "you" confusing?

Not OP, but thy question evokes an odd pining for idiomatic use of the second person singular in general English.

It was very clear to me.

To respect their gender identity.

OP's partner may be non-binary or otherwise use they/them pronouns.

Because they want to.