「そんなわけで、君達は今日から連邦市民ってことになったからね」
『……ほぼ一月ぶりに顔見せて、開口一番『そんなわけで』もねえとは思わねえのか、あんた』
隔離室の強化アクリル板の向こうのライデンはいたく刺々しい声音だったが、当初のような警戒ゆえではなく、単に不機嫌なだけらしい。
86─エイティシックス─Ep.2 ─ラン・スルー・ザ・バトルフロント─〈上〉 安里アサト
Does the bold part literally mean "Don’t you think there is no 'therefore'"? However this meaning makes little sense to me. How should I understand it? What’s the intended meaning?
The literal meaning of そんなわけで is "for that reason" or "therefore", which is obviously inappropriate at the very beginning of any conversation. ねえ (=ない) in this context is "impossible; unsuitable; unacceptable", not "nonexistent". This も is essentially a mild version of は (explained here).
開口一番『そんなわけで』もねえとは思わねえのか
Don't you think (saying) そんなわけで at the very beginning is unacceptable?
However, そんなわけで, てなわけで or such is common at the very beginning of a TV program, a manzai bit, an online meeting, etc. See this. This is often completely natural and harmless, just as starting a conversation with "So" is often safe in English.
Still, this type of そんなわけで ("So, ...") is a fairly casual conversational expression, and it's not a suitable word used before telling important news like this one. His irritation is understandable because the first speaker's way of speaking is too casual compared with its content.
