Eh IIRC the way it works is that they alternate between hardware (tech refreshes) and software upgrades in the modernization path. IIRC (but I may be wrong) Block 2B was a software upgrade, while Block 3i was a hardware upgrade, then Block 3F will be a software upgrade, etc. There might be for example some hardware changes in a software upgrade but not much. The plan was never that the F-35 would be for example still using current IR sensors for its EODAS and EOTS all the way until the last one is retired in 2070 -- they know technology continues to advance and the F-35 program explicitly designs and plans around it, in contrast to earlier fighter programs.
What the article is talking about is regarding aircraft already delivered. Since Block 3F isn't yet released, existing aircraft are using an older Block standard so naturally need to be upgraded to have Block 3F capability. Though the article doesn't explicitly say so, I'm guessing the 26 aircraft needing software-only upgrades are Block 3i, for example, going on back toward Block 2B and older versions which will need the most extensive upgrades being listed in the article. After all, they've been delivering F-35's since 2008 or something -- there's bound to be some older ones that haven't been upgraded yet, not to mention some changes like the helmet that weren't necessarily with a main Block.