I don't think YABU at all, I used to hear this from a colleague who had a DH that worked long hours so the family could live a financially very comfortable lifestyle and facilitated her only working a few hours pw.
What I think is so important to recognise is that nobody should generalise or make assumptions about the situations of others.
I left an abusive and controlling ex-husband in 2014 when my boys were just 3 and 6. Long story short he was FURIOUS I dared to leave him and through pure rage applied to the courts for full custody. Because the abuse had not been physical every other sort of abuse but, he was careful not to create "evidence", the courts ruled 50:50 shared care. Devastated was not the word.
Not only was I left without my kids for half of the time but because of 50:50 ex did/does not have to pay any maintenance whatsoever. I was working 3 hours a week as a teacher when I left, he was earning in excess of £105k pa. I had nowhere to live so stayed with parents for 6 weeks until I found a damp and tiny two-bed to rent privately (my school were amazing and I pushed my hours up to 16 pw). Ex remained in the five-bed executive family home. We were in that place for five years until I met my now fiance and we rent a bigger three-bed together now. Financially things are obviously easier now I have my fiance but he's a low earner so it's still a struggle with two growing boys and no money from ex.
I guess what I am trying to say is that would you class me as a lone parent? Shared parenting with a controlling narcissist is impossible at times and if I'm totally honest I sometones envy the fact that I don't have the same freedom to make decisions about my boys that ft lone parents do.
However, I fully recognise that a ft lone parent would perhaps feel envious of the fact I have so much child-free time which had afforded me the opportunity to develop my relationship with my fiancè (who had no bio children) and to have some "me" time etc.
Conversely, I also at times feel envious of anybody who doesn't have to be without their kids for up to 11 days at a time-they're 9 and 12 now and it still hurts like hell.
It would personally offend me if a ft lone parent said to me that I am not a "true" lone parent because I do often really feel like I'm not a "real" mum and this isn't through choice. My many battles and difficulties are just different ones.
I guess it's all about a mutual respect for one another and trying not to make it a case of questioning who has the hardest parenting struggle if that makes sense?