One has to take the postwar military comments on the atomic bombs with a grain of salt — they were only voiced after the war was over, and in a moment where the atomic bomb was getting a lot of "credit" for ending the war. These military leaders, who had essentially nothing to do with the making of the atomic bomb, were both irked that this technological marvel was getting the credit (at the expense of their campaigns, logistics, hard work, etc.) and were afraid (not incorrectly) that the Truman administration was going to try to use the atomic bomb as an excuse to cut conventional military funding deeply. So bad-mouthing the bomb was a way to combat both of these things, and was common in the years prior to the Korea War (at which point attempts to cut US conventional military spending ended, and the generals found that they, too, could live with the bomb).

I just finished typing up a long answer on the question of whether the bombs were "necessary", so I won't repeat myself here, but it's a tricky question depending on what one means by "necessary." You can apply the same issue to whether Japan was a "military threat" — it is less about that, and more about "the goals of the allies with regards to Japan," which were not just about the present war, but about their future activity as well (they didn't just want them to be temporarily neutralized, they wanted it to be essentially impossible for them to be aggressors in their sphere of the world ever again). Keep in mind that a lot of the long-term strategic thought in this period was based around the perceived failure to convert Germany from an aggressive power to a peaceful one after World War I, thus setting their existing carnage in motion through short-sighted policy.

There were certainly people who thought blockade-and-starve-and-(conventional)-bomb was an option for further weakening Japan, but the conventional wisdom (then and now) is that nations don't capitulate without "boots on the ground." (In fact, if one views the atomic bombs as having ended the war — which is not how all historians see it — then it would be pretty much the only example we have of a major nation surrendering as a result of aerial bombardment alone. The Soviet invasion of Manchuria complicates this judgment.) The allies were aware that, cut off from their overseas holdings, Japan would begin to face even greater food shortages than they were already facing, and their ability to wage war would be pretty small without oil, steel, etc., coming in from abroad. Whether that would be more merciful than an invasion (or atomic bombing) depends on what effects you think that would have on the Japanese people.

But to your main point, they were deadly serious about invading. That was The Plan. That was how they were going to end this war if Japan would not end it sooner. But it is of interest that the entire plan was not actually approved. In the summer of 1945, Truman approved the invasion of Kyushu, the southern "large island" of the main Japan islands. He deliberately did not approve the later invasion of Honshu, the main Japanese island. The idea, apparently, was to "wait and see" how Kyushu went, in terms of difficulty, casualties, resistance, Japanese response, American popular opinion, etc. This was prior to the testing of the first atomic bomb and so it did not factor much into Truman's thinking (or the invasion planning).

So that is serious, but interestingly not fully committed. If the atomic bombs and Soviet invasion hadn't ended the war when they did, the invasion of Kyushu surely would have gone forward. Whether it would have been bloodbath that pro-atomic bomb supporters claim is unclear; it depends on what other battles you think it would have been analogous to (many of the generals planning it did not think it would be nearly as bad as the island-hopping campaign, because the terrain would be very different and give them many more options). The plan was always "bomb and invade," it was only after the bomb appeared to end the war that they changed the story to have been about "bomb or invade." If Kyushu had gone well, then Honshu probably would have followed, though that was still up to Truman and dependent on the changing context. But my point is just that it was certainly a "serious" plan. I repeat myself, but it was, literally, the plan.