I don't think it's hard at all to imagine scenarios where his other assets could benefit from a player like Signal competing with Facebook, even if Signal is non-profit.

This is similar to how Google Docs competes with Microsoft Office. It hurts Microsoft's cash-generation and thus benefits Google as they compete with Bing.

If Google had been even smarter (or poorer, so as not to be able to afford footing the bill for Google docs entirely on their own), they could have established Google Docs as a 501c3 foundation, benefitting from tax exemptions and donations from individuals and just dominated that foundation through large donations and putting the right people in charge.

This may or may not be an accurate analogy for Acton's true motives with Signal, but I just can't know the truth of it and am therefore not taking that risk with my cash.