Correct, it is not supported
This is directly listed from the CED (Customer Expectations document) that should have been provided with the printer.
Duplex (2-sided) Printing
As with any printer, duplex performance is not expected to match the
performance for single sided printing. Paper jam rates may be higher
than the rate you will experience for the same throughput material in
single sided mode.
Manual duplexing is supported for all media weights per selected tray,
i.e. 280 g/m2 from MPT (Tray 1) or 256 g/m2 from all other trays.
Auto-duplexing is supported (for units with auto-duplexer installed) to
220 g/m2 from any tray.
75-169 g/m2 (Plain Paper, Heavy Plain Paper, Thin Card Stock) for
media over 128 g/m2, side two may become more susceptible to skew.
In plain english, what that means is :
75-128GSM paper will duplex fine
128-220GSM will work, but the image may be shifted
220GSM+ needs to be manual duplex, which is the marketing term for print the first side, put the output back in the tray and print page 2 on the other side.
So, you can duplex it only if you lie about what you put in the tray, but it increases wear and tear, isn't supported, and might print side 2 crooked. YMMV