I'm glad you asked this question, because I wondered the same thing. I have an 88 that had some serious audio upgrades and it has a very similar material sprayed inside.

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before applying real soundproofing material.

Why go through the trouble of stripping it out when you can just spray over it/finish the spray job?
You just need to add weight to the metal panel to minimize its ability to vibrate/create noise.
That weight doesn't need to come in the form of stick-on pads.
Spray on sound deadener is a huge business and most certainly is "real", though I obviously can't say that's what your particular product is. But I can't think of any reason not to just spray on top of it and add to the mass.

 

Looks better too. Spray on is what I used from Rustoleum

 

Rust-oleum undercoating? Did you notice a difference?

 

Undercoating is not a great sound deadener.