That was an extraordinarily upbeat report on the status of the strategic arms reduction talks that President Reagan's START negotiator, Gen. Edward L. Rowny, delivered the other day. He said that despite the airliner incident, Soviet-American agreement on guidelines for reductions was within reach by the end of the year. Is this news too good to be true?

You don't have to look far for skeptics who will say that 1) Rowny is whistling in the dark of the airliner incident, 2) the changes he now cites in the American position do not ensure Soviet agreement, 3) the administration seeks only to show enough flexibility in Geneva to sway the next congressional vote on the MX, or 4) Rowny is advertising progress both in Geneva and on Capitol Hill in order to win the quiet battle for control of the negotiation currently being waged inside the administration.

Any or all of these possibilities, and some others, may yet turn out to have substance. I worry most at the moment about the fallout from the airliner. Reagan has made an effort to insulate the arms talks from the primary shock, but Moscow is not helping much, and aftershocks keep coming. In Moscow as well as Washington the hard line may be on the rise. The conflict over missiles in Europe is building to a climax, too.

Nonetheless, there is reason to focus on the most striking aspect of the picture painted by Rowny.

The big complaint that Reagan's American critics have leveled against the American START position is that it's "unnegotiable": it may suit us, but it asks too much too fast from Moscow. It is said that Reagan demands unfairly that the Soviets "restructure" their forces--yield their most important and expensive existing (land-based) missiles--in return for relatively modest trims in weapons that this country so far hasn't produced or deployed.

Rowny took on the "restructuring" charge. He said the United States had stopped asking the Russians to reduce those big, 10-warhead (land-based) missiles that most trouble American strategists and instead was now asking just that the Russians reduce warheads to an agreed "reasonable number": "we don't care how you reduce to get there."

He also said the United States was now ready to accept a slower, more flexible time period (8, 10, 12 years) for such reductions--a pace that would fit easily, he observed, into the rapid rate at which the Soviets have been modernizing their forces anyway.

Since this new position was put to the Soviets in START's fourth round, Rowny went on, the Soviets no longer claim that the United States is trying to force them into unsatisfactory reductions. I would be startled if we had heard the last Soviet word n the subject, but that's what Rowny said.

He also addressed the second front-- the first front?--of the administration's START battle, the pulling and hauling not in Geneva but in Washington, specifically, the attempt by a swing group in Congress to trade support on the MX for changes meant to make the American START position more negotiable.

Rowny ticked off three recommendations--a compromise on throwweight, special treatment for bombers, a move to small, single-warhead missiles--made by Rep. Les Aspin. He said the administration had already delivered on 21/2.

Aspin, when I consulted him later, did not quibble with Rowny's reading. But he said the administration still has a good way to go to produce a START position at once sound and realistic strategically and capable of attracting bipartisan support over time.

He and other swing legislators are obviously sensitive to the charge they have sold their MX votes for a mess of arms control pottage. A number of them have formed a little action group to work with and through the Scowcroft Commission to ensure they are not taken for granted at the White House.

Back to Rowny's upbeat report, which bears directly on key negotiating decisions facing the administration: Rowny and some others would go with the modified START proposal he publicized the other day. Support is strong in the State Department for another approach sometimes called "SALT 21/2." A third approach, centered on finding a common "currency" in which to measure the destructive potential of different weapons, is identified with retired Air Force Lt. Gen. Glenn Kent, now at Rand.

My sense of things is that if--big if-- Moscow and Washington can clamber over the airliner hurdle, a preliminary agreement could come into view, if--another big if--the administration can cope constructively with the formidable external pressures and internal tensions now at play. It may not be much but it's more than a lot of us would have thought.