how do devas protect metta practitioner?

[bridif1]

It's one thing to be rational and scientific, but it's no longer rational when one rejects anything that their limited tool kit can't explain.

Most of the times, when some people talk about "events without explanation", those events are actually "highly unlikely and with low probability of ocurrence".

Possibility and probability are two different things.

Most "unlikely" events are possible (or explainable under the current scientific paradigm).

I suggest to not understimate the power probabilities, big numbers and statistics. There are billions of humans beings, so there are chances for unlikely events to happen more often than not.

Please, notice the use of the word "most".
I doubt scientific knowledge is near to be complete.

Kind regards!

[SteRo]

It's one thing to be rational and scientific, but it's no longer rational when one rejects anything that their limited tool kit can't explain.

...
pervasion by metta modifes ignorant inclinations beneficially. Thus "protection". "devas" are just mythological narratives in the context of corresponding attenuated ignorance.

Well the "tool kit" obviously is able to explain as your quote shows. The explanation however might be not to your liking and - this is admitted herewith - it might not correspond with theravada doctrine.