Personally I don't think East Coast and Lothian Country will be getting tri-axles.
The wording in the article is:
"60 tri-axle Enviro400XLB double-decks, [and] 20 B8RLE with single-deck bodies".
"The majority of those will be for the East Coast and Lothian Country operations, although some of the Enviro400XLBs are for the Airlink 100" (my bold).
The word "those" I would interpret as referring to the most recent bus mentioned, which was the single-decks at the end of the previous paragraph. This is furthered by the mention of the "Enviro400XLB" in the middle of that sentence, not at the start, so I would interpret everything before the mention of "Enviro400XLB" as referring to something else, and everything after the mention of "Enviro400XLB" as referring to the XLB.
Personally I don't think the B8s currently at East Coast are in need of replacement just yet. Discounting those, there are only five other single decks at East Coast which could be replaced, the short 191, 192 & 193 used on the 121 & 123 (and 120 on Sundays only), and 195 & 196 (used usually only on the 120). I'm unsure if the Monday to Saturday loadings on the 120 are low enough to facilitate use of a short bus like 191-193 but if they are then I can see those five being replaced with five short-length B8s for fleet standardisation.
Therefore to meet the definition of "majority [of single-decks] will be for East Coast and Lothian Country" at least six and at most 14 would need to be for Country. I have no idea which Country routes are suitable for single deck operation but is it possible that the 72, 73 and 74 would go up to every half hour but, along with the 70 and 71, go down to single-decks?
As for the XLBs - with their slightly shorter length I wouldn't be surprised if 20 were allocated to Airlink instead of 15. That leaves 40. They could be put on the 26 which would release about 30 Gemini 3s to go to East Coast and Country to allow them to remove their remaining B9s. That would leave 10 new XLBs left over.
Alternatively they could go somewhere else, I'm not sure where else would need them.