The fact that is still turns on and responds to touch doesn't mean it's all ok. He didn't fully test the thing. Lets face it. If the phone were really even somewhat water resistant, they WOULD have advertised it. The fact that they don't is telling.
The fact that they don't is only telling that it isn't up to the "standards" set by the current competition most likely, which if you actually look up what they are defined by are well beyond that little "demonstration".
OnePlus is obsessed with number comparison in their advertising, and a lower amount of resistance to water, even if beyond the requirements of perhaps most users is going to look bad simply because it is lower.
The results of that video while certainly not exhaustive, or useful by anyone making comparisons to a rated competitor it useful to me to at least shows a high likelihood of some resistance, and at that beyond my expectations for exposure to water, at occasional rain exposure if absolutely necessary.
If a phone could not stand exposure to rain, then I would have expected it to fail even a brief submergence test, which the OP5 appears to pass.
For my usage scenario that appears fine, but even those higher IP ratings advertised by some do not take into account real scenarios, only controlled conditions in fresh water, a static depth and therefore pressure, and not using the physical buttons.
Even the manufacturers of watches commonly described as water resistant to a given depth advise against using the physical buttons as that can allow water penetration to the their movement.
It should be understood just what a stated resistance with an IP number actually is.
Controlled conditions not representative of what happens during a typical water exposure scenario.
For those with expectations to survive drops in puddles, pool etc a higher rating does encourage confidence, but has a lot more variables involved which that rating does not even attempt to test.