I don't think creole is that well defined either: from certain perspectives Late Middle English was a creole of Early Middle English and French, the border zone between when Late Middle English could easily be considered to have been a creole versus where Modern English is definitely not regarded as a creole is really tough to define with all sorts of weird answers (from "it was never a 'true' creole because England still had an army on paper during the Norman Conquest" to "it stops being a creole when you have an empire and colonies are building their own creoles of your language" and all sorts of other ideas).
Late Middle English was not a creole, and that is not a matter of perspective - it's just a descendant of Early Middle English. A bunch of Norman borrowings didn't change that.
A creole is by definition the descendant of a pidgin, a patchwork of words "glued" with some ad hoc grammar, that looks nothing like the grammar of the parent languages. A good example of that would be the Jamaican Patwa basilect:
* Dem a kuuk akara fi im (lit. "them are cook akara for him"; "they're cooking akara for him/her")
* Im a kuuk akara fi dem (lit. "him are cook akara for them"; "he/she's cooking akara for them")
Even if most words are clearly English (except akara - a fritter), the grammar looks nothing alike. It was rebuilt from the scratch. We can't really say the same about EME vs. LME, where there's a clear transition from one to another.
>from "it was never a 'true' creole because England still had an army on paper during the Norman Conquest" to "it stops being a creole when you have an empire and colonies are building their own creoles of your language" and all sorts of other ideas
The presence of an army or "metacreoles" is irrelevant. Kreyol for example would still be a creole, even if the Haitians built a thassalocracy.
What matters is the presence of a linguistic community, that kept speaking their language as they always did. Normans only replaced the local nobility, but the Germanic speakers were still there - speaking among themselves in their Germanic varieties, even if they had to butcher a "pig" or a "cow" because of some fancy noble wanting "porc" or "beof".
