Not really. The tech cost is for timing. Maybe relic think the AEC needs the T2+side upgrade to make this unit not come out too early because that will cause 222/251 has little time space.
So the tech cost should not be add to the unit cost. Another example, will you add the 200/50 to the comet cost? The comet will be 700/235 then. It's even more expensive than a tiger/IS2. But no, the 200/50 is just tech cost to make the comet not come out too early.
While you're right for general tech, it doesn't apply here the same way.
For all factions tech is a way of timing units to the field, but you can progress from that next battlephase, tier building or truck and transition into another one.
For brits it doesn't apply, because you're simply going for a side tech and hitting a dead end.
Inferiority of AEC to puma assures no one will ever spam them and for the very reason of dead end side tech we can easily add tech costs to the unit, we can't when we talk about its performance, but as long as all strats involving it pick no more then single one ever, then tech cost can be added to "to get" cost.
Your comet example is similar, but not the same as comet is end tech tank and you will want multiple of it, in addition you're getting abilities and passives with this tech, AEC side tech is just that, mandatory side tech that increases units cost.
If it was like it was in alpha, where the tech cost was bigger, but AEC was actually affordable and you could build a strat around getting 2-3 of them then it would be a different story, but it is how it is.