If a player is the first receiver in midfield then they are, by extension, in a holding midfield role and part of the rest defence, in position to control opposition counterattacks. If a player moves or rotates into position as the first receiver in midfield, they also assume the responsibilities associated with that area in the event of a loss of possession. Neves rotates in and out of that area, spending constantly-varying periods of time in a holding midfield role over the course of a game. I think it's probably fair to say that he spends the majority of his time doing so. And when he is not in said anchoring position, his vacating of said space has happened in conjunction with the other midfielder rotating in and assuming that position and the accompanying defensive responsibilities.
The point with regards to Florentino Luis is that he probably spends more time in advance of Joao Neves, and it is therefore inaccurate to describe him as being the more positionally disciplined holding midfielder. Also, though I did watch one game where Joao Mario lined up alongside Neves, I believe that was a rare occasion and in the other games I've watched he has always been in the attacking band of three, usually on the left - Kokcu was the other player who operated in midfield. Regardless of which two were deployed there, the responsibility to sit and maintain structural integrity was shared among them. Sure, Neves played with a certain freedom - but that freedom was shared with the other central midfielders too.
Also, he thrives within far more areas of the game than the average player - for someone most readily categorised with midfielders whose strengths lie on the ball, he excels in many facets of out-of-possession play too. Mainoo is similar to Xavi, Modric, Kroos and the likes - a willing and adequate defender who will do his part but isn't going to provide excellence against the ball. Neves is different. More complete. While primarily a playmaker, he also shines when the opposition look to break quickly or have established possession.
Hence why, while I maintain it is not an ideal setup, I think a Neves - Mainoo partnership is certainly workable. Either share the workload entirely and allow the two of them to interpret the game and position themselves accordingly, or designate Neves as the 6 to Mainoo's 8 and still allow them the freedom to move and rotate as they see fit. Mainoo has shown his comfort receiving in deeper areas and even in his very good cameo for England as the 8 with Rice as the 6 there were instances where I felt he, and the England side, could have benefited from him rotating into Rice's position as the deepest midfielder to better aid in building the play. Operating alongside someone comfortable as both a 6 and 8 allows Mainoo to showcase his ability to do the same.
I am less keen on the idea of having Neves and Mainoo play ahead of a DM, due to both players excelling in deeper areas, as either a first or second function midfielder. Basically, I haven't seen either of them display the same comfort in more advanced positions ala Iniesta or Lampard. And even with a 4-3-3 and two 8s as opposed to a 4-2-3-1 with a 10, ultimately to have a functional setup one of those midfielders still has to occupy a higher position on the field.
The obvious caveat here is that they are very young and my opinion is based on limited viewing, so there is scope for them to develop into players who thrive in higher areas or that they indeed already are more comfortable in an advanced role than they have shown when I've seen them. I just prefer to look at what I currently believe them to be.
Everyone is extrapolating based on that. It's still just football they play in the English Premier League.
Very odd to pick out those three examples if your focus really is the physical and ball-winning aspects of the 6 role. While Rodri is a big unit and provides steel, the other two certainly excel more in the aspects you accuse the poster you are replying to of focusing on - positional discipline, taking the ball off the centre backs, safe passing option. They rely on their intelligence and have no aggression in their play. Busquets could probably have been blown over by a particularly stiff breeze. Neves is much more combative. Chuck a ball in the air or drop it in for a 50-50 challenge and Neves is a much better bet for coming out better off.