Hardibacker seems to draw moisture out of thinset?
The actual HardiBacker installation instructions (page 5) for tile do say to wipe the surface clean &quot;with a damp sponge&quot;:

Wipe the surface of HardieBacker® cement board clean with a damp sponge just prior to adding mastic or mortar.


This 2006 forum discussion also seems to anecdotally back up that wetting HardieBacker first is a known &quot;must do&quot;:

Hardi is thirsty, thirsty, thirsty. It tends to suck moisture out of thinset NOW and makes working time difficult. Kinda like laying wallpaper on un-sized plaster walls.

and

Dry backerboard will suck water out of mortar like a sponge. If too much is removed the final bond becomes very weak.