And is this preventing theft in what measure? Data at hand?

I'm having trouble parsing what you mean here. Could you elaborate further or use different words?

Thanks!

Does any data exists that can prove such countermeasure is lowering the rate of theft for Apple devices only, or is it just speculation?

Lots of data. Police departments in San Francisco, New York and London saw pronounced drops in iPhone theft rates shortly after Activation Lock was introduced. https://archive.nytimes.com/bits.blogs.nytimes.com/2014/06/1...

Thieves have taken to threatening the people they stole from because they simply aren’t able to bypass the activation lock: https://appleinsider.com/inside/iphone/tips/do-not-remove-ic...

While a device could be scrapped and sold for parts, many components in the iPhone authenticate with each other, which makes it even more challenging to usefully sell the parts. The same authentication has been criticized as obstructing legitimate device repairs, although Apple has been (dragged into) better supporting self-service repairs more recently.

Yes: https://www.zdnet.com/article/fcc-report-apple-iphone-activa...