Basically they are intending to limit their own options of how subtitles can be displayed. The limit of not being able to position multiple subtitles at specific position is the biggest thing imo. To go with the first screenshots the post has:

For the first it is translating the info box on the left and just adding it above its dialog subtitles. This has a few problems. First the text overlaps the text in the box, not the worst thing here but that can sometimes make things hard to read and imo doesn't look good.

Second it can make it hard to know what is being translated if there are multiple text fragments on screen. Take this screenshot from the article https://daiz.moe/content/crunchyroll/dumbell-funi-4.jpg how are you supposed to know which belong to which muscle? Or this where only one of the rows is translated and you don't even know which https://daiz.moe/content/crunchyroll/mha-funi-hulu.jpg

Third with two dialogs to translate (like tv in background plus people in foreground) you could probably better indicate where what is coming from with the ability to position it.