Like they plan to enforce it

Please be serious.

The Saudis bend over backwards to lip-service the US line. They have never really followed any US promoted policy domestically (unless you count "keep the oil taps open" as domestic, but it really is foreign).

Many US-aligned officially-sharia countries have high ages of marriage. Few enforce them, especially outside the cities where pesky American diplomats are.

Call me back when Unicef congratulates Saudi Arabia on their successes against child marriage. Then you will have a point.

Hoping the remaining recalcitrant sharia countries to actually enforce ages of marriage is like hoping that the most recalcitrant KP countries start enforcing KP possession and distribution laws. ie, not impossible, and even probable in the medium term, but still very far from having arrived at that point.