I don't want to venture into the legal question, because I think it depends on the details of exactly what they changed compared to the original, and how much human creativity went into those changes, and I don't think we have those details.
But, most engineers/PMs/etc don't have a good understanding of copyright law – it wouldn't surprise me if the authors of that blog post just slapped "Copyright Meta" on it by default because they are used to doing that, and aren't thinking at all about technical legal questions of copyrightability. Furthermore, it isn't really their job to think about those questions – that's what companies employ lawyers for – and I imagine the lawyers likely think that asserting copyright over the uncopyrightable has minimal negative consequences, whereas failing to make that assertion can work against them if it ever becomes the basis of a lawsuit, so better just tell the employees to slap a copyright notice on everything.
I once contributed (on my employer's time) to a FAANG open source project (I'll avoid saying which project or FAANG because I don't want to publicly embarrass anybody). I added a brand new file which I'd written from scratch; I copied the copyright/license notice from one of the existing files to the new one, but I changed it from "Copyright [FAANG]" to "Copyright [MyEmployer]". The FAANG employee who ran the open source project objected to that – "why did you change the copyright, everything in this project is copyright by [FAANG]"– the project didn't have a CLA, by the way. I told them they were wrong about the law, and if they didn't believe me, ask their own lawyers – and maybe they did talk to them, because they dropped the objection and ended up merging it, complete with my employer's copyright notice. So even FAANG engineers can fail to grasp the basics of copyright law.
But, most engineers/PMs/etc don't have a good understanding of copyright law – it wouldn't surprise me if the authors of that blog post just slapped "Copyright Meta" on it by default because they are used to doing that, and aren't thinking at all about technical legal questions of copyrightability. Furthermore, it isn't really their job to think about those questions – that's what companies employ lawyers for – and I imagine the lawyers likely think that asserting copyright over the uncopyrightable has minimal negative consequences, whereas failing to make that assertion can work against them if it ever becomes the basis of a lawsuit, so better just tell the employees to slap a copyright notice on everything.
I once contributed (on my employer's time) to a FAANG open source project (I'll avoid saying which project or FAANG because I don't want to publicly embarrass anybody). I added a brand new file which I'd written from scratch; I copied the copyright/license notice from one of the existing files to the new one, but I changed it from "Copyright [FAANG]" to "Copyright [MyEmployer]". The FAANG employee who ran the open source project objected to that – "why did you change the copyright, everything in this project is copyright by [FAANG]"– the project didn't have a CLA, by the way. I told them they were wrong about the law, and if they didn't believe me, ask their own lawyers – and maybe they did talk to them, because they dropped the objection and ended up merging it, complete with my employer's copyright notice. So even FAANG engineers can fail to grasp the basics of copyright law.