No. Acceptance of a filing and payment of the US$15,000 fee create no employment authorization. Work permission comes from lawful permanent resident status on admission, or from an independent authorization already held. If neither exists in March, the start date has to move, not the reasoning around it.
Testing the March start date against a real authorization
Take the two possible answers seriously. Either a currently valid status permits this specific employer and role, with dates covering March, or it does not. A pending Gold Card case adds nothing to that test.
Approval is never guaranteed, and neither is timing, so an offer built on an expected decision date is an offer built on a forecast. The safer choice is a start date tied to the document itself. Hypothetical example: a diagnostic laboratory accreditation consultant is offered a start date conditional on the file being accepted, and the employer proposes to keep the role open for sixty days.
That offer can be accepted on those terms, provided the start date is tied to a document rather than to a receipt. Ask the employer to write the condition as the production of a specific authorization, and ask what happens to the role if it is not produced. Both answers are better had in writing before a current position is resigned than discovered afterwards.
A sixty-day hold is a commercial promise and can be withdrawn.