وَيُسْتَحَبُّ أَنْ يَغْتَسِلَ قَبْلَ الْوُقُوفِ.
Translation: It is recommended to perform ghusl before the standing.
The word yustaḥabbu explicitly classifies the bath as recommended. Missing it does not by itself invalidate the standing.
The distinction between required and recommended acts is important when congestion, lack of facilities, or health conditions make bathing difficult.