I think you underestimate how much MIRI agrees with FLI.

Agreed - or, at least, he underestimates how much FLI agrees with MIRI. This is pretty obvious e.g. in the references section of the technical agenda that was attached to FLI's open letter. Out of a total of 95 references:

• Six are MIRI's technical reports that've only been published on their website: Vingean Reflection, Realistic World-Models, Value Learning, Aligning Superintelligence, Reasoning Under Logical Uncertainty, Toward Idealized Decision Theory
• Five are written by MIRI's staff or Research Associates: Avoiding Unintended AI Behaviors, Ethical Artificial Intelligence, Self-Modeling Agents and Reward Generator Corruption, Problem Equilibirum in the Prisoner's Dilemma, Corrigibility,
• Eight are ones that tend to agree with MIRI's stances and which have been cited in MIRI's work: Superintelligence, Superintelligent Will, Singularity A Philosophical Analysis, Speculations concerning the first ultraintelligent machine, The nature of self-improving AI, Space-Time Embedded Intelligence, FAI: the Physics Challenge, The Coming Technological Singularity

That's 19/95 (20%) references produced either directly by MIRI or people closely associated with them, or that have MIRI-compatible premises.