وَإِنَّمَا تَظْهَرُ ثَمَرَتُهُ حِينَ يَنْفَعُ النَّاسَ.
Its fruit becomes apparent when it benefits people.
Thamaratuhu borrows the language of a tree’s fruit for an outcome or benefit, suggesting growth and eventual yield. The temporal clause ties the image to a concrete criterion: benefiting others.
Innamā adds a restrictive emphasis, but the point in this debate is practical benefit rather than an assertion that every possible form of private or theoretical knowledge is worthless.