I would. But in general I would consider anyone a founder of a company if they had a significant impact before the companies first product/service went general availability to their target demographic.

If they already had a general release of product or service. I wouldn't consider them a founder if they joined afterwards.

Is Zuckerburg the founder of Instagram? The company went from 13 employees and 30 million users to 450 employees and 2 billion users. 98.5% of users came after the purchase, but I still don't think most people would call him the founder.

That's a bit disingenuous though, it's like saying YouTube got 99% of their accounts post acquisition, knowing that the majority of these were merely Google accounts who through the associative property of acquisitions were delegated these accounts by the purchaser.

In my opinion Instagram sold a bit early[0], but coming from an app that existed merely as the first widely used iteration of phone-camera filters, I can't necessarily blame them. Who'd have thought that Instagram, and also Giphy to an extent had -any- form of business model or potential for profit? I sure didn't.

[0] - sidenote: (in the financial and growth sense, not the chronological sense)