I think its ok to use spell names to imply a world.
I just think they've done a terrible job at it. The implied characters don't have any character. You can add character, but that's kind of a stupid way to go about things. The whole point of implied character is that its supposed to imply something. Its not supposed to imply that something could, theoretically, be made up to fill in a detail of the game world.
And yeah, yeah, I know that Mordekainen has some lengthy history in some dusty book that isn't going to be explained in the 5e phb. That makes it irrelevant to this conversation.
Greywiki tells me that Mordenkainen developed a spell entitled "Mordenkainen's Defense Against Nonmagical Reptiles and Amphibians." This does not imply a game world. And the addition of a completely empty proper noun at the front of that spell title does not do much except imply that there are other wizards, that some of them make up spells, and that they suck at naming things.
You can't even add up all the spells Mordenkainen ever made, and deduce things about his character. There's no thematic connection between them.
Its just not well done. Maybe it was, once. Maybe it was... once. When everyone knew Greyhawk, cared about its lore, and wanted to play games in it. But now we're just hearing the decrepit echos of an earlier era, rasping unwelcome in unexpected places.