それはいささか唐突だ。の英語

That is rather unexpected.

That is an extreme prejudice.

That is a horrible bias.

That is a clear [flagrant] breach of the rules.

That's beside the point.

That is beside the point.

You've got your priorities backwards.

That's a stupid question.

That's a stupid question!

That is troublesome behavior.

That is a stupid mistake.

That's forcing things.

That was a blunder, to begin with.

That's a mere fancy.

That is a studied insult.

That's a delicate point.

That's completely beside the point.

It's a bundle of contradictions.

It's full of contradictions.

That's absolute nonsense!

That is an obscure remark.

That's quite absurd.

That is quite absurd.

That's total horseshit.

That's flat nonsense.

It's a bundle of contradictions.

That is exactly two birds with one stone.

That's going too far.

If I had to say, it is fairytale-like.

If I had to say, it is whimsical.