There's a big difference between an elite swimmer *looking* effortless and them being truly effortless in the water.
As a case in point, our real-life Mr Smooth Jono van Hazel, who competed at the Athens Olympics in 2004 in the 50m freestyle, looks effortless when swimming at 1:10 per 100m pace. And yet this is actually his 1500m race pace - not something he can sustain all day. If it was truly effortless, by trying a little harder he'd be able to challenge Sun Yang's 1500m world record, which of course he can't.
The word effortless has been used in a well meaning manner by coaches for many years. The idea being not to fight against the water but work with it, as elite swimmers like Jono do. However, many swimmers have latched onto the notion that swimming should be truly effortless and as such they should never work hard in training or feel anything but very easy when they swim.
When you are swimming it should not be without effort. Just like when riding or running, you can feel strong, smooth and rhythmical. You are working efficiently with the water but it's never truly effortless. Swimming wasn't meant to be that way.