Shorter lockout bonus - anything other than Gribbles?

The distinction isn't raid vs. group. It's DZ lockouts vs. task lockouts. DZ lockouts show up on the list when you hit Alt +Z. Most raids use these, but also some group content, like MPG group trials. Task lockouts show up when you enter /tasktimer, and I don't think they are affected by this bonus.

According to the post on the main page it is for instance lockout timers which includes all tasks and expeditions. I think they either worded it wrong or didn't implement it properly.

"It's time for another round of Fall Fun bonuses!
From Wednesday, November 15, 2017, until Tuesday, November 28, 2017, ALL players will experience 50% shorter instance lockout timers while adventuring!
You'll want to gear up and get out there - half the lockout time means that you can have twice the adventures!"

If raids are affected, then one thing you can do is farm for overhaste items from old raids like Anguish and Beltron (10% and 12% overhaste clicky I think).

They seem to be less common drops, so the more chances you get the better.

Both soloable / groupable I think (I know omm is at least)

I really liked the named spawn rate boost. Another day, we got 4 grand herbalists in a row plus dry rot, deathfang and an overseer all in space of 1.5 hours.
If you have a group thar can reliably do hero missions, knocking out all four and then doing them again shortly after can net you lots of T2 gear and nice augs.

If it at least would work like intended. But quests/missions in TBM are regularly from time to time not able to spawn a chest. Every third/fourth quest/mission simple locks without spawning a chest...
So for people hanging on some older quests this bonus is simply useless...
a double xp or a faster named respawn would have been much more useful.. perhaps it is possible not to mess up the system with that...