When they say it takes extreme training, it's an understatement. Longbow wielders had to begin training in childhood, to develop the necessary muscles. If you started too late, you wouldn't be able to do it. And yes, training took decades.
That's why firearms became so widely used. Because you can train any idiot to fire a gun in a short amount of time. Of course, nowadays with modern technology, we have things like crossbows and compound bows that don't require a huge amount of strength in the right places to wield, but they are still a lot more difficult than a gun, and modern firearms have orders of magnitude more penetrating power than even the best bows.
Something like this takes a massive amount of training and muscles in the right areas (which have to be developed over a long period). Deadly in the right hands, but if you give one to someone that hasn't had extensive training they will probably just hurt themself.
Something like this takes much less, but it's still a lot harder to effctively use than a firearm. Modern materials like aluminum and carbon fiber can make the bow itself much stronger, and much easier to draw and shoot.
And then these, even more power and even easier to use, but still not as effective as a firearm.