No they are not. No oil is too thick for an engine regardless of the clearances, an engine can be designed to tolerate lower viscosity oils but that in no way precludes the use of thicker ones.I believe from what I have gathered and read across multiple Toyota and Lexus forums. The 2010 and newer RX 350's 2GRFE are spec'd specifically for 0W 20 possibly due to tighter ring landing clearances. What's speculative to me is whether the 2009 year was tooled abd fitted to operate in the 0W 20 range rather than the 5W30. Depending on where it was produced, assembled, and what time of the year in 2009. It has been suggested but not confirmed. Glad you are having a good experience though. I may consider that.
The only time an oil is too thick is when the winter rating is inappropriate for the starting conditions and it gels in the presence of the pump pickup tube. In that case it cannot be pumped and the engine will fail due to oil starvation. Otherwise if it can be pumped it will lubricate.
If some oil was too thick for an engine at operating temperature imagine what any oil would do when cold.