“A DTG printer reads a 30% transparent pixel as a command to lay down a 100% solid white underbase dot”

Printify's underbase guide says it in almost these words: a 30% transparent pixel gets a 100% solid white dot underneath. That is one RIP's behaviour stated as the printer's. Epson's Garment Creator operation guide, the software that drives Epson's own DTG printers, describes the opposite for its white layer: the amount of White ink decreases depending on the transparency, which is how it prints white gradations such as clouds. So a semi-transparent pixel can get full white or graded white depending on the RIP and its settings. Printify's rule is a safe instruction for sellers who cannot see the RIP, not a description of every DTG printer.