FileAttachment(...).csv() returns a Promise. Observable’s Runtime resolves Promises automatically for you, but only when they are referenced from other cells/blocks.
If you want to consume the value of the Promise in the same cell you have to await it:
const a = await FileAttachment("./data/trimmed.csv").csv({typed: true});
Inputs.select(d3.group(a, d => d.Aggressor))
or
const a = FileAttachment("./data/trimmed.csv").csv({typed: true});
Inputs.select(d3.group(await a, d => d.Aggressor))
Not at all! 