Extraordinary claims require extraordinary evidence. The reality is that UFC divisions are meat grinders that typically house 90%+ of the elite fighters in the world. As such we have a strong evidentiary base when it comes to exercises like ranking a top five or top ten, for the simple reason that these guys compete against one another with regularity. Their encounters provide us with the evidence. If, on the other hand, a fighter does not participate in that meat grinder because they do not compete in the UFC, then the evidentiary base is much smaller and projections about rankings swiftly become divorced from evidence and lapse into simple speculation. I remember reading on this forum that Vitaly Minakov was perhaps the best HW in the world. In reality he may not even have been in the top ten. He certainly wasn't top five. Moreover, when people leave the UFC and go elsewhere they are typically already in decline.
In Lima's case, while he is clearly very good indeed, we just have no clue how we would fare against elite guys in their prime. In the Strikeforce era there were still genuine conversations to be had, but the concentration of talent in a UFC that runs 40 shows per year means that not only am I generally unwilling to rank a non-UFC champ over the UFC champ, but I'm very wary of ranking them even in my top five. I would need to see super compelling evidence first. There are exceptions of course, for example Demetrious and Horiguchi.