TDD is done in a tight loop (minutes) while coding. For every little micro-feature/fix, you write a test for the new behavior you want, implement the minimal ugly fix to get the test to pass, then rely on the tests so you don't regress as you clean up.

LLMs struggle with TDD. They want to generate a bunch of code and tests in large passes. You can instruct them to do red/green TDD, but the results aren't great.

SDD starts before implementation, and formalizes intent and high-level design. LLMs eat it up. The humans can easily reinvent the worst parts of waterfall if they're not careful.

They're not mutually exclusive.