My bet for Timestream is that is built on top of DynamoDB. There are many potential indicators (1KB writes, throttling) and some clear ones(pricing follows the exact proportion up to the 4th decimal if you compare across regions, for example), it supports "unlimited" scalability, batching, etc. That reads are eventually consistent may be because they are computed over a GSI. It would be interesting if true as it is cheaper for writes than DynamoDB (on demand, which is the model Timestream has).
Plus there is a reasonable amount of mindshare and possibly market opportunity into offering time series on a serverless database like DynamoDB.
If this were true, it would also mean that Timestream, as hinted in the post, is more performant when accessed more in parallel (as DynamoDB itself is by design a massively parallel multi-tenant infrastructure).