but at the time he was still a Jedi, albeit one that preferred the shadows. Agen was deliberately kept in the dark about the plan, so his responses and attitude would be genuine.
Luke serving Palpatine in Dark Empire was also intended to be a ruse.
Yes, nothing contradicts Dooku and Kolar being in the same teir, but the sample size is very small.
It's true that the sample size isn't large, but no matter which way you look it's still ultimately supported.
For instance, Saesee Tiin, who was implied was an inferior swordsman to Kolar, was capable of dueling Mace Windu to a draw in a sparring match between the two. While this does not mean Tiin was necessarily equivalent to Mace as a lightsaber duelist, it does support the insinuation that a duelist superior to Saesee could possibly be approaching Dooku and Mace.
Another example is Kit Fisto. He was able to solidly overmatch General Grievous, who has both challenged Dooku in their sparring sessions and fought Mace to a standstill at one point in time. Earlier in the Clone Wars, Obi-Wan openly admitted inferiority to Fisto after a sparring match between the two. Again, it supports that Kolar, who is even more talented as a swordsman than Kit, could definitely be approaching the higher tier duelists.
If the artist had drawn that fight in a different way, would you still have the same opinion?
Probably not, but the comparative nature of the two duels made it much easier to compare them realistically.
I'm not claiming Kolar's still is equivalent to Dooku's or even to Obi-Wan's, but he's definitely just around the corner. People tend to adopt the misconception that the main characters are on a league above their contemporaries, which is simply unsupported.