The decoction doesn't extract tannins, therefore a hot sparge won't extract tannins theory is debunked.
Decoction mashing extracts more tannins than an infusion mash. Along with gelating the starch, boiling the mash extracts husk compounds, including polyphenols (tannins). The level of tannin extraction, however, is fairly low and some maintain that this low level actually benefits the flavor of the beer. If a low, pleasing amount of tannin extraction is a piece of “decoction mash character,” then simply adding Munich or melanoidin malt would not capture that character exactly.
Homebrewers used to infusion mashing may wonder how a decoction could be boiled without extracting a large amount of tannins and yielding a very astringent beer. After all, when lautering, they are repeatedly told that their grain bed temperature should never exceed 170 °C (77 °C). The key to understanding this apparent discrepancy is understanding when tannins are soluble in wort. Increased heat and increased pH both favor tannin extraction. At lower pH values, such as those found in a thick mash, tannin extraction from grain husks is minimal even at boiling temperatures. At higher pH values — such as those in a grain bed that has been extensively sparged — excess tannin extraction occurs at a much lower temperature.https://byo.com/stories/item/537-decoction-mashing-techniques