This is true, but it's significantly harder to encircle a unit AND kill/suppress all of its fighting strength (to force retreat). Unless a unit is extremely vulnerable or already damaged, you need a lot of firepower to accomplish this. At minimum, 2-3 effective attacks in a single turn (usually some combination of artillery, air, and direct assault). For particularly robust units, like Heavy Tanks, it might be 4-6 attacks before you've neutralized it enough to force a retreat.
Also, you keep using the word "envelopment" to mean "completely surrounded," and that is not the meaning of that word in a military context. The military word for "completely surrounded" is "encirclement".