The Flesh Tearers Strike Force is a Detachment using a
FOC, and it's rules apply to everything in that Detachment. The Formation is a separate Detachment, and it's own rules apply.
The only way a detachment's rules can apply to formations is when you're taking one of the newer Mixed Detachments. Detachments like the Gladius Strike Force from Codex Space Marines is a detachment made up of a mix of individual units and Formations, so in that case the Formations within it DO benefit from the rules of the Gladius.
Here's an infographic I made to try and explain everything - the FTSF is a 'Faction-specific
FOC' -