The article just summarizes another article (https://www.carbonbrief.org/solar-is-now-cheapest-electricity-in-history-confirms-iea) which actually reports the IEA findings. It turns out that solar is only cheaper if you factor in the special government programs which give super-favorable below-market financing to utilities to build solar, and shift default risk to the taxpayers:
"This shift is the result of new analysis carried out by the WEO team, looking at the average 'cost of capital' for developers looking to build new generating capacity. Previously the IEA assumed a range of 7-8% for all technologies, varying according to each country’s stage of development. Now, the IEA has reviewed the evidence internationally and finds that for solar, the cost of capital is much lower, at 2.6-5.0% in Europe and the US, 4.4-5.5% in China and 8.8-10.0% in India, largely as a result of policies designed to reduce the risk of renewable investments."
Solar remains substantially more expensive than other technologies. Some governments subsidize it enough to make it competitive, and this analysis relies on that.