one example is warlock's summon imp. timer is 8sec when in reality its 10

Which update, classic patch 1.13.7 or latest addon update?

Casts with talent speed reductions such as warlock summons are not accurate since you can't detect other people's talent. With 2/2 Master Summoner its a 6sec cast time.

It's been like that for all of Classic. If you target yourself it should show correct cast time, but not if you target other players.
Kinda surprised you're seeing 8 sec cast though, it should be either 6 or 10. Are you targetting yourself while having 1/2 in Master Summoner talent?

I just remembered it always being accurate before patch 1.13.7. Same for some other spells with wrong cast time that I can't remember on top of my head but used to always be correct. Is casts with talent speed reduction not accurate always a thing or is it something new after 1.13.7?