You can give someone the amounts of equity and salary which are normally given to cofounders, but that doesn't make them a cofounder. The word "cofounder" doesn't say what you do; it says what you did -- specifically, it says that co-founded something (in this context, a company).
If I incorporate a company one day and bring you on board the next day with half of the available founders equity then would you argue that you're "just an employee" because you weren't there on the day that the company was incorporated?
When did attempting to maintain consistency in how the English language is used become playing semantic word games?
... would you argue that you're "just an employee" ...
No, for two reasons. First, I wouldn't say just an employee, since I don't consider that word to be derogatory; and second, I wouldn't stop at saying that I was an employee (unless I was deliberately attempting to obfuscate) because that would fail to mention a large equity stake. I would probably say "early employee", "co-owner", or something else along those lines.
But if the company was founded without the intention that I would be part of it, I would never claim to be a co-founder.
In practice, founding a company is a process, not an event, and hence is an interval of time, not a specific point in time. Legally, founding a company is considered an event at a given point in time, purely for reasons of convenience (how do you tax something that came into existence over a period of time?)
If you look at it that way, we're coming from the practical standpoint, not from a legal standpoint, and we feel that on the practical side we're still founding the company. If you look at it this way, poof, the inconsistency disappears like a cloud of smoke :)
Sometimes using a label that is technically incorrect is the most succinct way to make a point.