Here’s the example sentence in one of my reviews:
つまり、その病気が感染症ではないということなのである。
what does the なの actually add? Is this 100% synonymous with ということだ？ Considering that である is just more formal than だ.
According to this explanation なの just adds a bit of emphasis. But I’m not sure if it’s somehow especially important in this set expression here. (ということなの)
TL;DR: と言うことだ vs. と言うことなのだ
I’m pretty positive that なの here is just doing what んです・のです does. So no, this is not 100% synonymous with ということだ because the speaker, when using なの is adding emphasis, but also it means that they’re explaining something to someone.
This is somewhat covered in the description for ことなの where なの, like んです is simply emphasizing the point and conveys some explanation.
In this sentence, it’s most likely that someone asked a question looking for an explanation, and so the speaker is now explaining it to them, which is why ということなの is used.
The difference is one has のだ and one doesn’t. You can think of the な as a necessary part to join them or if you want to understand structurally then だ turns into な. なのだ goo Dictionary
と言うことだ + のだ => と言うことなのだ