I don't doubt there were some impressive defenses, but this seems to me like looking back with rose colored glasses. (This sounds more antagonist than intended. Sorry)
All of those linebackers would be a-okay in a modern 3-4. "Pure" 2-gapping schemes perhaps not, but now? They would do just fine. Which is kind of what I'm harping on. There are certainly differences between a 4-3 and 3-4 but they have far more similarities.
As far as the 46 defense...I'd argue it has more in common with 3-4s or 4-3 under (which is sometimes called 3-4 with 4-3 personnel) that a "true" 4-3. I'm contradicting myself slightly when I talk about pure vs. not, but I'll do my best to explain.
The 46 starts by shifting the line to the weak side. This starts to create an odd front. The strongside DE is now aligned as a 3T DT, you have NT on the Center, and the other DT is also a 3T. The weakside DE ends up not involved with the shift. He's staying on the outside shoulder of the tackle.
On the strongside of the formation, both players that would be called the outside linebackers are aligned on the LOS to the tight end side. To keep the straight, I'll call them Tacklebacker and Endbacker. As aligned, it looks kind of goofy, but how it makes sense in my head is think of the Endbacker being locked up with the tight end. Unsure how common it was in 1985, but let's pretend the tight flexes and becomes a slot receiver...and the Endbacker goes with him.
All of a sudden, you have 5 men on the LOS...that sure sounds like a 3-4. The two men off the LOS are Singletery and Mr. 46 the Strong Safety. Singletary, interestingly, is not aligned in the middle of the defense, but he's shifted to the tightend side of formation. Somewhere between the guard and tackle. He's your scary, angry man that can deal with lead blockers.
Mr. 46 is aligned over the weakside of the formation. Over the tackle. Playing a safety/linebacker hybrid role. Not unlike what many 3-4s have been trying, even Green Bay (admittedly unsuccessfully) with players like Josh Jackson.
So 5 men on the LOS, two split linebacker guys not aligned over the center. One snot nose player to the strong side, one smaller, faster, playmaker to the weakside. How is that not a proto 3-4?
Snark aside, the 46 had far more in common with modern, single-gap 3-4s than its contemporary 4-3s. Or 4-3 over defenses (ala Jimmy Johnson). One could even see this. A few times under Capers, the Packers had a "bear" call, which shifted the linemen in (both ends became 3Ts), the OLBs widened their alignment, the SS came into the box...stop me if you've heard this one