The automatic extension applies only if the Plan Sponsor has extended their corporate return and is only until the due date of the corporate return. So, the extension may only be until 9/15. Using this method is more of a fallback method if the Form 5558 filing is accidently missed.
It is a more conservative approach to file the Form 5558 Extension rather than rely on the corporation return being extended.
In their comment letter to the IRS dated May 30, 2025, ARA recommended that the deadline be extended by default.
https://araadvocacy.org/wp-content/uploads/2025/06/ARA-Government-Affairs-Comment-Letter-2025-05-30-Notice-2024-202.pdf