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The shortest version: Solid 2 makes reactive relationships part of the running program. Octane tries to keep application code closer to ordinary JavaScript while giving a compiler more responsibility for understanding and optimizing it. The debate comes down to what reactive code should mean when you write it, and what information should remain available when the app is running.
The thread began as a developer-experience scorecard. After several rounds between Dominic Gannaway, creator of Octane, and Ryan Carniato, creator of Solid, the disagreement has become both narrower and more interesting.
It is not really compiler vs runtime. Both use both.