In this reading, in is for negation, and lammā denotes the exception.

In this reading, in is emphatic (lightened from inna) and la-mā reinforces this emphasis.

Abū Jaʿfar al-Madanī

Ibn ʿĀmir al-Shāmī

وَزُخْرُفًا ۚ وَإِن كُلُّ ذَٰلِكَ لَمَّا مَتَـٰعُ ٱلْحَيَوٰةِ ٱلدُّنْيَا ۚ وَٱلْـَٔاخِرَةُ عِندَ رَبِّكَ لِلْمُتَّقِينَ