This is not accurate. "Validate Mobile Compatibility" on a "Free Form" Landing Page Template does very, very little. In fact, it only does the following:
Must have <!DOCTYPE HTML>
Must have a <HEAD> element
Must have a <TITLE>
the <HEAD> element
Must have <META CHARSET=
> within the <HEAD> element
Must have a <BODY> element that contains one (and only one) <DIV class=
So, it doesn't add practically anything to your Free Form template's code. It ensures that the above points are in place so that if a user chooses to "Activate a Mobile Version" of a Landing Page that uses your template, Marketo can make it work correctly and build their basic, mobile view. Marketo does not add any classes to a Landing Page unless a user Activates a Mobile Version of that Landing Page. Just because a Free Form template is "Mobile Compatible" doesn't mean a user must activate a Mobile Version on an individual page itself that uses your template. It can be left de-activated (which is also the default setting).
You shouldn't instruct people to never activate a mobile view of their "Free Form" page. Free form templates are generally not built to be responsive, so without activating a custom mobile view built within the Free Form landing page editor, most pages would not look good on mobile.
For templates that already have responsive code built-in, the vast majority of our customer now leverage "Guided" mode landing page templates (>50% of Marketo Landing Pages are now Guided). The Guided landing page editor does NOT have a "activate mobile version" feature because these templates, by nature, are responsive.
For customer that have coded responsive Landing Page templates, the Guided mode pages should be leveraged.
You're correct about the Validate Mobile so I've changed the message. Upon further investigation, someone I work with opened this in the LP editor and turned on the Mobile version which is how the code was inserted, then never informed me that they had done so which is why I made my hasty jump to a conclusion last night.