“Send DTF files in RGB, because converting to CMYK first tends to dull the brightest colors”

DTF Transfers Now (Source 1) says to send RGB, not CMYK, because its printers are profiled for RGB and an earlier CMYK conversion tends to dull the brightest colors. Transfer Superstars (Source 2) says the opposite: convert to CMYK with the FOGRA39 profile before submitting. Each is a shop's own rule for its own workflow and neither is a general authority, so the RGB advice holds for the shop in Source 1 but not for DTF printing in general, and the claim omits that.