You hit the post, so to speak, and I too had forgotten the 10xxx and 20xxx fleet number phase so it was an interesting rabbit hole to disappear off down
After so long of having fleet numbers almost exclusively between 1 and 999 (usually singles 1-200 and doubles 201-999, with numerous exceptions), it was quite an interesting time indeed having 4 and 5 digit fleet numbers on so many buses, with new buses almost exclusively given entirely new numbers (the initial ex-Londons onwards) rather than taking numbers freed up by withdrawn buses. It would seem that these days we're mostly back to the old system, however 1000-1153 and the coaches (9001-9013, 9101-9102 and 9201-9208) remain with 4 digit numbers. With the size of the current fleet (including Eve coaches and trams as well) being large enough that sticking purely to 1-999 could be difficult, so I assume we'll continue to have 4 digit numbers in the furure, at least for coaches if buses are now back to 3 digits for new batches.
I believe that the 5 digit fleet numbers in the early years of ECB were due to systems in the depot being configured for 5 digit fleet numbers of the previous operator, so they just added the prefixes of 10xxx to singles and 20xxx to doubles.
I would take a guess that new batches will be assigned up to realistically 998 and then restart at 301, or maybe 351 or 401 if they want to leave space for future trams.
On the subject of fleet numbers, I am curious to see if the Lothian Motorcoaches fleet and Eve Coaches fleet merge to the point of having just one livery and fleet number system, if they'll go for the Eve style Exx numbers or the LMC 9xxx numbers, or something else entirely.
As to the ex-Arriva Eclipses, 101 and 102 were always with ECB so iirc only carried the 10101
(01010101...) and 10102 numbers, while 103-107 went to Lothian Country and officially numbered just 103-107 without the 10 prefix. Then there were ex-Whitelaws 193 and 194, 10193 and 10194 at ECB, which where then renumbered TB107 and TB108, meaning that within the 2010s, Lothian technically had 3 seperate Wright Eclipse Urbans numbered 107. Them being 107 - SN04 NGX, TB107 - SF55 HHC (RIG 6498) and 10107 - GN06 EWD (RIG 6497).
Also I believe that 942 only carried the 20942 number for a year or so, because when it returned with 779's roof it was back to just 942.
It is an interesting phase, and could potentially be confusing to someone in 3 or 4 decades time who wasn't alive in the 2010s, researching the history of the fleet and suddenly encountering stuff like 20001 and 10101 among a fleet numbering system otherwise well established and fairly simple.