This is not correct.
14 CFR 121.436(a)(3) establishes the requirement for 1000 hours of SIC, in order to act as pilot in command under Part 121. Specifically:
https://www.ecfr.gov/cgi-bin/text-id..._1436&rgn=div8
In context of this thread and your response, experience obtained under Part 135.243(a)(1) speaks to more than just aircraft with 10 or more passenger seats. The language of the regulation addresses three possibilities, separating them with commas and using "or" to verify that any of the three are applicable to that subparagraph.
135.243(a)(3) specifically points to:
- Turbojet aircraft
- Airplanes having a passenger seating configuration of 10 or more
- Multi engine airplanes in commuter operations
https://www.ecfr.gov/cgi-bin/text-id..._1243&rgn=div8
Any of the three are applicable to the requirement to hold an ATP under Part 135.
A pilot flying a turbojet aircraft as PIC under Part 135 must hold an ATP.
A pilot flying an aircraft with a passenger seating configuration of 10 o more (excluding any pilot seat) as pilot in command must hold an ATP.
A pilot flying a multi engine airplane in a commuter operation as defined under Part 119 as pilot in command must hold an ATP.
This is to say that the SIC requirement may be met, in context to your statement, by PIC experience under Part 135 in the above aircraft. Also, in accordance with impending changes already announced as a final rule in the Federal Register, experience as SIC under 135 will also be applicable.