HiddenPower13 | Super Mario Sunshine - Developer oversight freeze @HiddenPower13 | Uploaded January 2014 | Updated October 2024, 3 hours ago.
Looks like they simply forgot to reprogram the pokey sprouts so that they just don't produce coins even when being smashed against the wall.

Then again, they created 100 shine spots for other impossible episodes, so why not this secret? I guess they didn't feel the need to because not a single yellow coin is supposed to be obtainable here.