This program targets acid rain. It only applies to coal and diesel. No one is building off-grid coal power plants.

Diesel generators are the standard for power generation where a grid connection is not readily available and/or the power needs are temporary (e.g. emergency backups). Cheap, easy logistics, and durable. This use case has always been allowed in practice because there is no practical alternative. Many industries rely on diesel generators for reliable site power.

It doesn't apply to natural gas power plants such as those used in data center builds at all because they don't produce this type of pollution.

I fail to see the big deal. This guidance appears to clarify the language so that it matches actual practice and intent.