New EPA regulation requires coal plants in the United States to reduce 90 percent of their greenhouse pollution by 2039 (gifted link)

Gas is a different energy source than coal. It has different characteristics both environmentally and health-wise. EPA has already signaled that they are skeptical of methane gas as an energy source for environmental reasons and the current administration has halted permitting of new gas plants. But the article does say that future (hypothetical) gas plants are seeing new regulations, which will limit future emissions:

Under the new regulations, future natural gas plants that generate electricity at the rate of at least 40 percent of their maximum annual capacity would have to reduce their emissions 90 percent by 2032. New gas plants that generate electricity at less than 40 percent of their maximum annual capacity would be required to use low-polluting technology, such as energy-efficient turbines.

Emphasis is mine. The purpose of this rule's application to future gas projects is to discourage utilities from switching from coal to gas. The goal is to get them to switch from goal to something actually green.

As for the coal plants, this regulation compresses the timeline for 90% emissions reduction by a year (from 2040 to 2039). That is not "nothing." Remember that an EPA decision that would immediately put an economically significant corporation out of business would, generally speaking, be overturned in court just as quickly.

But technically, the paragraph you're looking at is misleading. The real timeline is this:

Under the plan, coal plants that are slated to operate through or beyond 2039 must reduce their greenhouse emissions 90 percent by 2032. Plants that are scheduled to close by 2039 would have to reduce their emissions 16 percent by 2030. Plants that retire before 2032 would not be subject to the rules.

Emphasis is mine. The regulation compresses the categorization of plants with different life expectancies, but still presents a timeline for the near future. Plants that expect to exist for a long time have until 2032 (that's 8 years, not 15) to get to a 90% emissions reduction. And obviously that is not going to happen all at once; it will be incremental over the next eight years. That means that in as little as, say, one or two years, we could start to see some reductions.

The article also writes:

The E.P.A. also imposed three additional regulations on coal-burning power plants, including stricter limits on emissions of mercury, a neurotoxin linked to developmental damage in children, from plants that burn lignite coal, the lowest grade of coal. The rules also more tightly restrict the seepage of toxic ash from coal plants into water supplies and limit the discharge of wastewater from coal plants.

That is significant. Again, hardly "nothing." And this part of the regulation would go into effect much more immediately than the emissions limits.

It's worth reiterating that the overarching reason the EPA has to take such an incremental and narrow approach to regulation is that, as the article mentions, the Supreme Court restricted the EPA's ability to mandate a move away from coal.

What are you expecting? National infrastructure change take time. Yes, decades for stuff like this.