This my be true now, but when I was working and doing the marketing for my products, it reported accurately. I'm pretty sure it still does honestly.
According to Customer.io, the change was to make images load by default, not requiring the end user to click Show Images. Thus making the open rates more accurate.
AFAIK, it doesn't. It will proxy all images through its own servers (and thus hide your IP), but it happens as your browser requests it and is not cached.
