It's too early to say if President Reagan's silken speech to the United Nations, a speech disguised as a disarmament offer to the Soviets, will reassure west Europeans, to whom it was really addressed, about the December deployment of 572 new missiles on their soil.

But already we know it did nothing to advance the consensus on arms control he is said to be seeking at home.

Hardly had his words faded off the screen when retired Rear Adm. Gene LaRocque of the Center for Defense Information was saying at a downtown news conference that the president's new proposals were "preposterous" and "absurd."

The Europeans professed to be thrilled that, at long last, Reagan had said a nuclear war is unwinnable.

They had been considerably shaken by Alexander M. Haig Jr.'s "demonstration shots" and Caspar W. Weinberger's survivability studies.

"Better late than never," sighed a British diplomat at the glass palace on the East River that arouses such ambivalent feelings in Reagan.

But LaRocque said, "A nation has two policies, its stated policy and its real policy. Reagan now says he is for arms reductions, but he is planning to build about 17,000 new nuclear weapons, and that is his real policy. Nothing he said in New York would reduce the number of weapons or eliminate any system we are building."

The Center for Defense Information, an association of retired senior officers who are at war with the nuclear buildup, has, through laborious research, gathered hints and clues from congressional hearings, old buddies and often little-noticed public statements. It estimates that the Reagan administration will spend $450 billion in the next six years on its 17,000 new weapons.

Dr. George Rathjens of the Massachusetts Institute of Technology, who once worked on the hydrogen bomb and who now spends his time on the anti-nuclear side, joined the "MacNeil-Lehrer Newshour" on Monday night to call the president's new proposals "cosmetic" and "empty."

He spoke with particular disdain of the supposed "flexibility" in the U.S. position on negotiating on bombers, something we have hitherto steadfastly refused to do. Rathjens says that the disparity in claims--the United States says that 500 of our bombers should be counted against 3,000 of theirs, while the Soviets say roughly the exact opposite--makes the offer meaningless.

"We are so far apart, it was safe for him to throw it on the table, without risking any serious negotiations," says Rathjens.

Nor is he impressed by Reagan's suggestion that he might cut back on the number of Pershing II missiles in exchange for some unspecified Soviet reductions in their Intermediate Nuclear Force.

Rathjens is not sure he agrees with the inference that the Soviets are more concerned about the Pershing, which can hit a Soviet target within eight minutes of launch, than about the cruise missiles, which can evade radar.

The offer to understand that Soviets have missiles aimed at China while suggesting we must have missiles to offset them, Rathjens finds "very ambiguous."

Like everyone else, Rathjens thinks the Soviets, in their handling of the Korean airliner incident took more steam out of the European peace movement than anything Reagan said at the United Nations.

"They made it possible to put those stupid missiles in Europe," he said.

The day after Reagan's speech, a bipartisan trio held a news conference to present their arms control plan, which has nothing in common with Reagan's new proposals, of which they could find little good to say.

"They have little chance of becoming the basis for an agreement," said three alumni of the Arms Control and Disarmament Agency and previous nuclear negotiations: Paul Warnke, former director of ACDA; Gerard C. Smith, SALT I negotiator, and John Rhinelander, legal adviser to the SALT I team.

Their starting point is a U.S. offer of an "indefinite" postponement of the deployment of the European missiles in exchange for a Soviet reduction to 600 weapons in their Intermediate Nuclear Force, which is where they were before the Soviets started their 1977 buildup.

Reagan is unlikely to accept any delay in the deployment of the 572 missiles in Europe.

He has now moved the argument from the point where it was a year ago. Then people were questioning his curious notion that producing more nuclear weapons increases the prospect of peace. Now he is putting it about that the Soviets will negotiate only when the missiles are in the ground.

And if he believes that, you may well ask, why does he fling around new arms proposals now and expect them to be taken seriously? The answer is that he doesn't. He just needed to say something flashy to the United Nations.