So in TOS they were at about the same technological level. ENT revealed that they had access to a constitution-class ship like 100 years prior. Basically even with that boost they were still barely able to keep even with the Federation in the normal universe.
Then the Terran Empire falls. We see that the Klingons are about at the same technological place as the normal universe. However, despite being previously basically static in their technological advancements they made a huge leap to still be even in technology with the current Federation. They regrew their power and advanced.
My problem is... how the heck did they manage that? The Terran Empire hasn't been too good with technology in the past. What logic has given them access to basically exactly the same ship classes? Have they had operatives on our side for decades copying our tech? That's the only thing I can think of that would make sense.
I don't see the technologically challenged Terran Empire who's been broken and re-pieced together still being at an even level as the normal Federation. They're pumping out the same classes. Basically the same tech. It's a bit too much suspension of disbelief for me unless they've just been stealing the main universe's technology all this time.
In STO they call themselves the Terran Federation, not Empire.
I think new weekly series with them should answer these questions, or maybe a proper story arc in Season four. They kind of just show up with little explanation other than that they tried to invade our space with the help of the True Way and that presumably there is a power vacuum left from the removal of O'Brian by his son and then the removal of the son by us.
The Terran Empire in Star Trek doesn't make sense.
It was a fun single story episode in TOS which was expanded out of proportions in DS9 and Enterprise.
Its so ludicrous that that any explanation will do.
After the rebellion against the Cardassian-Klingon alliance the formed the new Terran Federation used phased transporter technology to spread their influence and evilness. They used this technology to jump into other continuity where a Bajoran-Federation conflict had weakened the federation to the point that the Dominion was able to sweep aside all opposition. In one of its last ditch gambles starfleet of this continuity used captured dominion technology to improved the warp plasma efficiency of all its ships, with a side effect of tinting its nacelles a deep purple color.
This along with the stolen schematics for all current federation starships allowed the Terran Federation to assemble a vast fleet again. This combined with the conquests in weaker continuities allowed the Terran Federation to grow to a strength mighty enough to conquer our Alpha Quadrant. Muahahahahahahaha... *Strokes Goatee*