Is that speculation? I think it only uses elo to determine opponents and not ranks. When they added ranks I didn't notice any change in the matchmaking.
You've mentioned only the 1-20 ranks, nothing about their connections to elo, which they are not.
The ranks have completely nothing to do with matchmaking, they are simply an indicator where you are more or less on the ranking of your faction in regards to your factions full population.
Ladder rank 300 on faction with 1000 players will have different 1-20 rank then ladder rank 300 on faction with 10000 players as 1-20 rank is your relative placement within your faction, something like "context" of your ladder rank.
Another example, you can be rank 300 yourself in low population faction and be matched with rank 500 in high populated faction, you both could have similar elo(or not) and say rank 16 as that rank is your relative position on your factions ladder taking into account all active players of your faction.
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Relic Entertainment
