The World Bank classification is for High-Income, not developed. Even Russia, Mexico, and Turkey all caught up to that definition at various points in time before regressing. Even China's GDP per Capita is closeish to that level yet median household incomes and HDI are still lower than Thailand and Serbia.

Czechia didn't catch up to Western Europe HDIs until the mid-2010s, and Median wages were significantly lower than much of Western Europe until recently (eg. The datapoint above), and GDP per Capita is still significantly behind much of Western Europe and the Asian Tigers.

Even South Korea didn't officially become developed until the mid-2010s, and they outpaced Czechia in HDI and GDP per Capita by the early 2000s.

Alternatively, if Czechia was a developed country in the mid-2000s, then Turkey and Hungary are now developed countries in the early 2020s as they have hit similar indicators as Czechia in the 2000s.

> Czech lands were pretty developed

Developed versus developing is a technical-ish term.

Czechia was absolutely a developing country until recently. Just having a medieval university or some industrial capacity in the early 20th isn't saying much, as Korea and Taiwan were in a similar boat as well under the otherwise brutal Japanese colonial rule.

And even today, water treatment in Czechia lags significantly behind much of the Western EU member states.

Saying otherwise is just Eurocentricism.

And it also ignores that fact that for most of it's history, it was CzechoSLOVAKIA and Slovakia is still lagging behind on developmental indicators to this day, despite having a similar linguistic and ethnic proximity to Czechs as Serbs are to Croats.