No, M1501.1 clearly does not allow discharge to the soffit
"The air removed by every mechanical exhaust system shall be discharged to the outdoors in accordance with Section M1506.3. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space."
I agree with Mr. Grimm. Additionally, M1506.3 states that openings shall comply with Sections R303.5.2 and R303.6. The exhaust opening sizes found in R303.6 would not be provided by vented soffit, and therefore must be vented to required grill or louver. Additionally, R303.6 requires the termination to be outdoors, which would be in opposition to terminating the opening within the soffit confinement.
My opinion is that the person asking the question believes a bathroom exhaust grill through a vented soffit meets the intent of "exhaust directly to the outdoors", I do not. I believe the intent of the code in 1501.1 "to avoid exhausting contaminants into... concealed spaces as attics...where moisture can damage the buildings components." would not allow a bathroom exhaust vent to terminate through a vented soffit because it could not avoid exhausting contaminants into attic through soffit vents adjacent to bathroom exhaust termination grill.
