The OM is published to civilians (for the least common denominator), to those who acquire them for SD but never train with it much less clean them.
Fire a.357 revolver with alum and you’ll find the alum extracts harder vs. brass, 9mm/35K-psi against alum should pose no problem expanding/sealing the casing (including steel) in the chamber. The 9mm stepped chamber is another proof of HK over-engineering.
HK recommends against alum/steel... because alum corrodes to a rough surface, steel is coated and may be Berdan primed. HK has no control on how you maintain ammo, so HK C-Their-A stating it ‘could adversely affect safe and reliable functioning’.
You’ll also find some say HK’s are designed for 9mm/124. HK’s ‘are designed to fire quality, factory-loaded ammunition, loaded to NATO or SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute) specifications for use specifically in handguns’, that’s 115/124/147gr.