Baka malaglag lang sayang..mostly 1/4" thick lang ang ginagamit na plywood sa wall..

Kung matatapat sana sa posisyon na gusto mo..

I locate mo na lang yung skeletal frame(pamakuan) kadalasan 2"x2" lumber yun,dun mo na lang i screw yung wall mount holder ng tv mo..

... tama si, you need to mount the mounting bracket of the tv on the wall where the frame supports the hardiflex is located to ensure that the wall (and its frame) can carry the weight of the mounting bracket + the tv.

the problem now, from my understanding of a tv full motion mounting bracket, is that it allows you to move the tv forward a few inches away from the wall an i think you can also swing the tv left or right or tilt it up or down. doing all this will put more pressure to the connection of the mount to the wall kasi, malayo na yun tv from the wall, unlike kung naka dikit lang yun tv to the wall using a fix mount system.

the frames (either wood or metal rails) of a hardiflex wall are normally space 2' from each other because the size of the plywood is 4'x8'. and most likely, the base (where you put the screws to secure it to the wall) of your tv wall mount is smaller than 2'.

if the above is all true, then one way to attached the tv mount (normally has 4 screws for mounting) to the wall securely is to use 2 pieces of wood (or metal) that you mount horizontally to the wall where you want to place the tv mount. the 2 pieces of wood should be long enough (2 feet) to reach 2 frames where it will be attached.
to be on the safe side, i would recommend to extend the 2 pieces of wood to about 4' so that you can secure it to 4 frames of the wall - it may not look nice to but it's more secure at baka nga bumigay kung sa 2 frames lang naka secure.

once the 2 pieces of wood/metal are installed horizontally on the wall, you can mount the tv rack on it and then mount the tv to the rack ... cheers

Boo!