This is how I treat it as well. I go as far as letting the LTU nobles be LTU nobles, CT nobles be Golden Age nobles, and T5 nobles be Galaxiad nobles, and switch around based on what setting I'm playing.
I have Galaxiad-era nobles. I'm running two CT adventures at a con this summer that are technically in the year 1900. But single-world adventures seem to be quite setting-agnostic.
That is certainly a reasonable way to interpret the T5 Noble system relative to earlier rulesets in different eras.
However, there is a way to "harmonize" the two systems to a certain degree (especially if it turns out that T5 is retconning everything in all eras) based on how the land grant THexes are allocated (or for those who simply like the new T5 system and want to use to for their earlier-era campaigns).
For example, lets take a Count. Under pre-T5 nobility he was associated with 2 or 3 world clusters (with no particular indication as to criteria for assigning such clusters, and not all worlds were part of such "counties" anyway), and was given a land-grant of less than or equal to 10,000km
2. Under the T5 ruleset he is associated
primarily with a single high-population world (usually), and has 32 THexes to be scattered across the fief-world's sector (and 1 LHex per THex of personal property).
My reconciliation would be based on how the THexes are assigned.
For example (and I will ignore the non-mainworld hexes for the purpose of the illustration), I might assign about half of the 32 THexes to the Fiefworld. Of the remaining 16 THexes, I might assign half of them them to 1 or 2 closely associated worlds (i.e. a "cluster"). Of the remaining 8, I might assign half of them to THexes within the fief-world's subsector, and the remaining 4 would be scattered about the fief-world's sector.
So in the end, the Comital title is associated primarily with one world (and we will make it a high-population world, if possible), but is also associated with a core of 1 or 2 additional worlds (comprising the 2-3 world cluster of pre-T5 nobility). The remainder of the THexes across the subsector, and (to a lesser extent) the rest of the sector, then represent simple off-world interests relative to the core-worlds, in declining frequency that farther one moves away from the fief-world.
It is not a perfect harmonization, but the point is that you can still create noble territories that look somewhat like the pre-T5 assignments using the T5 ruleset.