The reserved prefix $ answers the question: "how can we let people name, structure, and nest their tokens however they want, while future proofing the spec?". More on that rationale: https://tr.designtokens.org/format/#character-restrictions

As for the "pretty weird" aspect — it's definitely uncommon but it's been seen before: the $ prefix is also used by the JSON Reference ($ref): https://www.ietf.org/archive/id/draft-pbryan-zyp-json-ref-01... — and just in case, we ran this syntax past Ben Hutton (inventor of JSON Schema) to ensure we weren't doing anything silly.