Source code is one of my pet peeves also but for entirely different reason than GITG talks about. When I first started IT work in 1985, you always got source code when you bought ERP. And you usually had people like me who could customize it to the business and support it. The source code was in major business languages in common use like COBOL and RPG supported by vendor who had nothing to do with ERP vendor.
The exception was the source code in the security part of system, where they implemented code to prevent you from changing hardware without getting new "keys" from the ERP vendor so the software would work on new hardware. That forced you to pay annual maintenence or repurchase software again when changing hardware. I never did understand why these ERP vendors felt entitled to be paid again just because you changed hardware. Upgrading to new release is one thing, running same code on other hardware is not reason for them to cash in again. But they got away with it, giving them that steady stream of maintenance revenue forever.
But that wasn't enough for them. They wanted that lucrative action of customizing and deploying for companies that bought it. So they began using proprietary programming languages and compilers, limiting the pool of IT people who could work on it to them and their partners. The ones who still used "normal" programming languages just started withholding all source code. For legacy customers that had customized, they agreed to sell you rights to have this source code to support your customizations. This money is in addition to what you payed for the ERP itself, and it wasn't cheap.
The escrow GITG is talking about had to be for code to the security parts you never got. That gave you some protection in case vendor went belly up and your hardware upgrade introduced a major change, at least you could change/compile and keep going. An example of that is when IBM AS400 went from 32 bit CISC to 64 bit RISC systems. Programs had to be recompiled in most cases. So your ERP vendor had to send you new compiled security programs you didn't have code for. If vendor gone, you were 100% DOA.
These source escrow vendors may or may not have been any help in that situation. If simple recompile, sure. If any code needed to be refactored for any reason, probably not.
GITG is right, in long run if your ERP vendor goes belly up, you will find another solution. But it can take a long time to switch, especially if you have customizations.
So in the short run, which could be 1-3 years, you better have some protection to change hardware until you get to new ERP. And only access to source code can do that. But that still no guarantee, depends on how they lock their software to a piece of hardware, the generation of a key/license. If that algorithm is not in your source code, you are DOA no matter what. Well, unless you are really good at hacking hashing routines....