They actually had Julian Morris in their first issue. So I don't think they have problem with inviting celebrities. But how many will agree to pose is another question.
You are right about A-listers. Most of them won't even pose shirtless for the cover of Men's Health/Fitness magazines, let alone hoping them to pose nude for Yummy. I think the company usually ask celebrities posing for Man About Town or Rollacoaster instead.
I see your points in your post but I think how to define 'celebrities' is going to be another debate, just like how to define 'top male fashion models'. Probably each member here will have a different criteria. Let's just say 'agree to disagree' and let's not even go there.
I totally agree with you that Yummy is a different thing for each reader. I said this before, Yummy can disappear at any time. So I treasure this magazine and hope that it will be doing well and last for a long while.