> No. We make the source code available to enable you to fully customize your game, but your rights are limited by the Lumberyard Service Terms. For example, you may not publicly release the Lumberyard engine source code, or use it to release your own game engine.
That's a contradiction "FOSS" encompasses the software which meets the (nearly-equivalent in practical application) FSF Free Software definition and/or OSI Open Source definition.
If it is open source software (which this is emphatically and expressly not), it is FOSS.
Software can be commercial and open source meaning you can read the code but you have to purchase a licence to make copies and/or use it - that's open source but not Free[-Gratis] Open Source Software (FOSS).
Although FOSS has a broad definition[1], most technical people refer to Free Software by the GNU interpretation, in which the Free in FOSS is "Free as in Freedom, not Free as in Beer". FOSS is generally considered a subset of Open Source software.
You can charge as much as you like for FOSS[2].
Open Source is not the same as Source Available[3].
> Q. Is Lumberyard “open source”?
> No. We make the source code available to enable you to fully customize your game, but your rights are limited by the Lumberyard Service Terms. For example, you may not publicly release the Lumberyard engine source code, or use it to release your own game engine.
So according to them it's not even "open source".