Untrue, for lots of reasons. First, European states could have raided ocean going federal commerce and caused huge problems for the north. And those battles were fought more or less exclusively by wooden ships.
Then there's obviously Canada as a staging area to pressure the North, or Mexico.
And the problem with coming into Southern Ports was that they were blocked by about 300 Union ironclads.
There weren't 300 ironclads until very late in the war. There were none through 1861. A European fleet could have forced its way into any southern harbor it chose.
Plus, the vast majority of the union fleet, particularly the coastal blockade fleet, was made up of wooden ships. Ironclads were more common on the rivers because ironclads were crappy on the high seas.
So, had England and France both joined the war on the Confederate side, what would have happened? They would have put supplies and their armies onto rickety wooden boats and sailed them to America, where they would have been systematically blown out of the water at the mouths of Southern ports by Union Ironclads.
By what date do you consider every southern port to have been blockaded effectively by union ironclads? Because prior to that, wooden ships would have been very effectively, as demonstrated (in part) by the Union Navy's continued use of them throughout the war.
After Gettysburg and Vicksburg, and the election of 1864, the South was doomed, and no collection of European allies could have changed the result, because they couldn't get past the Union ironclads.
What about before Gettysburg and Vicksburg -- more than two years? European intervention then could have been decisive because the blockade on the Atlantic coast was not a solid wall of iron, and union merchant shipping could have gotten hammered. Would the North really have kept fighting if 1) the south freed its slaves, 2) New England merchants were seeing their ships and profits sink, and 3) the war still looked like it a lot more fighting ahead?
New England merchants DID see a lot of their profits sink. The Confederate commerce raiders were deadly in their effectiveness.
There is also a problem of size. By the middle of the war, the Union and Confederate Armies were enormous. The British Army was puny in comparison. The British were afraid for Canada, because the Union had such huge armies it could have pushed up there at will.
Also, by the middle of the war, the Union Armies were very experienced and had good weapons and supplies. The European Armies were not used to fighting this sort of thing. Bismarck and the Prussian General Staff learned a lot from watching the "armed mobs" in America fight. Everyone in Europe did.
Unit for unit, the Europeans were no match for the Union or Confederate Armies after 1862 anyway. And in terms of sheer size, the Union and Confederate Armies, with their million men, dwarfed the armed forces of old Europe. The Civil War was really the first think akin to a total war.
The Confederate Army was considerably larger than all of the forces on both sides of Napoleon's invasion of Russia.
Britain's Army in the 1860s, total, was in the 60,000 range. The Union Army was over a million. Europe would have had the upper hand in no sense here, and the Europeans knew it.
What you say about naval warfare is true. It's also true that the British admiralty wanted no part of a war with America. We can debate the relative merits of things, but they feared those ironclads. This was a quantum leap in naval technology which left everybody scrambling.
As far as counterfactual history, I do think the South could have won the Civil War, in 1861, immediately after First Manassas (Bull Run). The victorious Confederates could have advanced on Washington DC and probably would have taken the city at that point. The Union militia army had melted, and Union regulars were very few.
But the South didn't press that advantage at that instant. It would have taken balls of solid rock, and a different strategic vision to be sure, but had the Confederates surged north and Washington fallen, the Union government would have been in utter chaos. Maryland would have seceded, and Kentucky. Perhaps also Delaware. The whole balance of everything would have changed rather precipitously. In all of that political chaos, the Union would have had to have tried to reconstitute a government somewhere. And at THAT point, Britain and France may very well HAVE intervened, and that would have probably sealed the North's fate.
Which would mean that we would all probably be speaking German because without the USA, Germany would have almost certainly won the First World War.