I agree that simply having few friends, or struggling to maintain friendships, isn't in itself a symptom of autism. That would be far too simplistic, and it's not how autism is diagnosed.
For me, as someone diagnosed later in life, the important distinction is that the diagnostic criteria aren't about the number of friends you have. They're about a persistent pattern of differences in social communication and social interaction across contexts — including things like social-emotional reciprocity, interpreting and using non-verbal communication, and developing, maintaining and understanding relationships.
So the relationship part isn't simply “you don't have many friends.” It can include things like finding it difficult to know how to adjust your behaviour across different social situations, understanding the unwritten rules of relationships, making or maintaining friendships, or knowing how to navigate the back-and-forth of relationships.
And that's only one part of the assessment. For an autism diagnosis, there also have to be persistent restricted or repetitive patterns of behaviour, interests or activities, with at least two of the specified characteristics, alongside evidence that these differences were present in the developmental period and have a clinically significant impact.
That's particularly important for people like me who were diagnosed later, because I spent years assuming that the difficulties I had with people, relationships, sensory experiences, change, communication and so on were just personal failings, personality, anxiety or me being “too emotional”. Looking back, the assessment wasn't about finding one thing I couldn't do, like maintain friendships. It was about recognising a much broader pattern that had been there throughout my life.
So yes, I'd agree that saying “not being able to maintain friendships is a symptom of autism” on its own is an oversimplification. But difficulties with developing, maintaining and understanding relationships can be relevant evidence within an autism assessment. The important thing is the wider, persistent pattern and the context around it.