KTX2(and .basis) have this “Universal supercompression” Meaning that once downloaded, a worker thread transcodes them to a real compression format. Meaning it picks the optimal format for the hardware the person is viewing with and that is what’s sent back to main thread
The first article is talking about device specific the ‘older’ way. Which is you offline generate a version of a texture for each different CPU (mytex-astc.ktx, mytex-etc2.ktx, mytex-pvrtc.ktx etc etc ). Babylon for instance then letting you know which suffix to download.
KTX2 is definitely the modern and recommended solution. For ALL cases, like constantly/en masse` during runtime, the jury is still out, with some users at least 