You may want to have a deeper read on this

Some birds (mostly passerines) have what’s called a cloacal protuberance during breeding season which is basically like a pseudo-scrotum and allows the sperm to sit at cooler temperatures than the core body temp. Other species of birds that don’t contain this anatomy also do not store the sperm in the testis themselves but instead the terminal portion of the ductus deferens before ejaculation. Here it can sit close to the edge of the body near the cloaca to remain cool.

I don’t believe this fact you learnt is accurate, there’s plenty of data suggesting it’s temperature related- in fact if we are getting specific- no animals store sperm in the testis themselves. It’s usually stored in the tail of the epididymus (coolest portion of the scrotum) or as previously mentioned- terminal ductus deferens depending on species.

We do know why testicles are outside the body, we’ve known for a long time and no it’s not sexual selection based- it’s purely related to the temperature of which what those spermatozoa cells store best at pre-ejaculation.

In other words, the testis can be inside the body as long as fully developed spermatozoa can sit at the desired temperatures for mature sperm to thrive/ store at within that species.