It’s not more resistant to flow at higher temps. For example, a 5W at 0 degrees is ‘thicker’ than a SAE 30 at 210 degrees. Make sense yet?
A 5W30 oil flows like an SAE 5 when cold, and an SAE 30 when hot.
But an SAE 30 is ‘thinner’ when hot vs. a SAE 5 when cold.