Yep. Using “they” and “them” as singular pronouns is actually really common, but in it’s (until very recently) common usage, it’s usually an indirect singular pronoun, rather than a direct one.
I’m no linguist, so my terms may be a bit off, but when referring to a single person, or multiple people indirectly (without them involved in the conversation directly and/or, not talking directly to them). So for example, Joe went to fix the thing, and someone asks if the thing is being worked on… Yeah, Joe is on it, they will get it fixed.
That’s normal.
The pinch for most people, that they can’t seem to grasp, is that many seem to believe, whether they consciously realize it or not, that referring to someone as a “they” or a “them” directly is usually considered … For lack of a better term, rude. In the same vein as calling someone by their name but getting their name wrong. It’s impersonal which comes off, in their mind as insulting.
I’ll give you an example, Frank just did a stupid. While standing in a group with Joe and Frank, Joe says, “then they decided to do the stupid.”
Same with “they did it!” While accusing a singular individual.
The reason people don’t like calling someone “they” and “them” is because on some level, they realize that the language is either dismissive or accusatory of the individual in question. Akin to calling someone stupid or using an undesirable nickname for someone, like referring to them by their race, or doing so via a racial slur; this example is a bit extreme, but you get the idea.
There’s an absolute fuckload of examples of using they/them as singular pronouns, but people are still uncomfortable with it, often feeling like it’s wrong to refer to someone like that without really understanding why; and because they don’t understand why, they’ll never intellectually move past the taboo of it.
Non-binary people have reclaimed the word as their own, and have asked the rest of us who are comfortable with our gendered pronouns, to use these words as their pronouns. So while it feels wrong/insulting to do it, it’s actually insulting not to.