The reason why its 170m+ is because that the distance required for the enemy to temporary despawn so that the game doesn't lag out the players / to improve performance but for some reason all of the troops are linked to the fabricators of the outpost so if you destroy the fabricators outside the spawn range they all just permanently despawn at best, at worst some trooper on the gun emplacement that comes with the setpiece stays.
It is also the reason why the pin scanner from the scout armor can only scan enemy troops within 170m not because that the limit, because they were actually temporary despawn outside of player's range except for structures and gunship patrols.
However, enemy can choose to refuse to despawn outside of the 170m range if they are still in active/aggro mode and for some reason the normal troopers and commissars have way longer active time, that is why it's key to do so when the enemies are not even spawned in yet.
Examples of 1 enemy not despawning due to a previous aggro
The rule work for Jammer outposts, AA defenses, Mortar emplacements, Detector tower (eye), Regular outposts and you don't need to destroy all fabricators for them to despawn (if there are more than 1) as long you and other Helldivers are 170m + away.
Here are some examples:
just Jammer related examples because this is where the rule get the most usage from, the other outpost types and optional objective people tend to destroy it on the go with stratagems .
Special cases like Main objective bases: Command bunker bases, Orbital cannon bases, Airbase towers or Landing zone (LZ) require all fabricators to be destroyed for the enemy to despawn inside.
Although this rule doesn't find any practical usage for clearing Command bunker bases, it still bring huge value and time saving for Orbital cannon bases, Airbase towers and Landing zone (LZ) if you can find the angle on all fabricators, not all bases are born equal some variation of each bases are easier than other.
Here are some examples:
currently need example footage for Orbital cannon bases.