"I thank you" should be correct as well, shouldn't it? As 'you' is also plural in English
The "you" could be implied as singular or plural contextually but the "allen" forces it. So, for an English translation that has all ambiguity removed the "all" is required.
Why is there a comma? Don't you say "Danke euch"?
There is no reason for a comma. It's a mistake on duo's side.