There is none. The comment is from some random gmail account, it doesn't mean anything.

tsets are described in more detail here: https://buck2.build/docs/rule_authors/transitive_sets/. Bazel's depsets were one of the influences on their design. To users, they will seem fairly similar and would be used for solving similar problems, there's some differences in the details of the APIs.

I'm not well-versed on the internal implementation details of bazel's depsets, but one interesting thing about tsets that may further differentiate them is how they are integrated into the core, specifically that we try hard to never flatten them there. The main two places this comes up are: (1) when an action consumes a tset projection, the edges on the DICE graph (our incremental computation edges) only represent the direct tset roots that the action depends on, not the flattened full list of artifacts it represents and (2) when we compute the input digest merkle tree for uploading an actions inputs to RE, that generally doesn't require flattening the tset as we cache the merkle trees for each tset projection node and can efficiently merge them.