I wouldn't get her if you want a horse to ride. They don't say why she's companion only- I'd guess she's not broke to ride, and at 20 years old, I'd be inclined to leave her that way. It also says she was seized in a cruelty case, which means that she's probably had some very bad experiences with people and might be very reactive under saddle.
She does look like a nice enough horse if you're looking for a horse to keep your other horse(s) company. They don't mention any special needs, which is always nice.