It really not about only children IMO though and it's not just about inviting a friend along.
For me it's typified by WhatsApp messages along the lines of "Where are you? Tom wants to go for a bike ride and we can't without Freddie" or "Freddie has played with Alex every day this week and I've had to play football with Tom." (Left unanswered and followed an hour later with a request to send Freddie over after school) and also by the fact that Tom's mum drops Freddie like a hot brick when Tom's dad is home from working away.
It's not wholly one sided of course because the "companion" child gets taken places, but it's the obligation and almost feeling the other parent feels they have joint custody that sets it apart!
DS is friends with the boy whose mum does this, but he also has a fairly all consuming sport club commitment and several closer friends - we live within sight of the family who have decided DS1 is partly theirs so the mum often sends a WhatsApp as soon as we get into the driveway and it is stifling - I've been borderline rude to her about backing off and she does for a while but it ramps up again as soon as her DH goes away for work because she finds her son hard work and it makesher life easier if she borrows mine too. She has told me numerous times how easy I have it with 3 kids and that they have stopped going on family holidays because their ds gets bored and it's such hard work (she has not invited DS on holiday though and he would not choose to go if they did - I would potentially let him go on a theoretical holiday with a theoretical friend if he himself was keen, but not this friend as his mum is very hard work).
I think it is all about parental personality not about only children - I'm sure the same dynamic could develop in families with siblings who don't play together due to big age gap or just having nothing in common.