Hi @Oscars, thanks for your interest in Jake. Good to see that you're focussed on nutrition. I'm happy to explain.
We've chosen for pea protein as our main protein source for a couple of reasons. First and foremost our choice for pea protein has to do with the amino acid profile. As you've pointed out, pea protein is often not considered a complete protein. It does provide all essential amino acids, but it's not complete in the sense that it doesn't provide enough of certain amino acids.
And that's exactly why the amino acid profile of pea protein is perfect for Jake. The amino acids that pea protein misses are cysteine and methionine, which are abundant in oats - the main ingredient of Jake. The peas and oats together form a perfect protein profile.
Should we have used any other source, say soy or rice protein, we would've overshot on these amino acids. Normally, that wouldn't have been an issue. However, cysteine and methionine are special in the way that they're the main sulfur-containing amino acids. We actually don't want any more of them than is necessary (search for methionine restriction if you'd like to read more).
As this reply is already becoming an entire book on its own, I'll keep it short on the further reasons to go for pea protein: It's vegan, it's hypoallergenic (no soy, no lactose, no gluten), it's a slow digesting protein and it fits the flavour.