So my FR235 5ATM = Fenix5 10ATM?

Ussualy this is "Standard table" when chekcing water resistance:

I have no idea what you mean by "ussualy"(sic). It's neither a technical standard nor an industry standard. Individual manufacturers are free to publish their own specifications and/or guidelines as to for which activities they deem their product models are suitable. What you as an individual consumer usually choose to consult as a rule of thumb is irrelevant.

That means garmin really has some high tech seals so FR235 can handle snorkeling just fine?

It means Garmin is prepared to accept that wearing a FR235 while snorkelling does not constitute misuse, for the purposes of product warranty. The technical implementation of the water resistance rating may well interest you, but is nevertheless not the point.

And I'm interested, since jumping into water is allowed, will I be fine with FR235 jumping "on the head first" (so my fingers and then my watch hits water first so START and BACK buttons are exposed to high pressure water ) from like 3-5 meters high?

Go back and read the Garmin page you quoted. It states, "You should note that even if a device is above a depth it is rated for, it might still suffer water ingression if it is subjected to an activity that creates pressure on it that exceeds that depth rating."

So if by Garmin 5 ATM activities = 10 ATM activities, whats the deal here if you know what I mean?

You can slice a tomato with a paring knife, a utility knife, a carving knife or a chef's knife of a given brand, but that does not mean all the knives are the same; and, there are some things you can cut with a chef's knife but not a paring knife. A little two-door hatchback or a monster four-wheel-drive/utility vehicle may both be OK to be driven on suburban roads, but that does not mean the hatchback will survive off-road driving through rough terrain (or that a 4WD/SUV wouldn't).

Garmin may have chosen not to clearly state which activities are safe for 10ATM devices but not for 5ATM devices, and that is its prerogative.

I've had my watch for 1.5 years and I go to the pool nearly every day. This includes diving regularly from 3M board and to at least that deep. Also, regular hot tub and sauna use. I have never had a problem yet!

Max

Tnx both for answers.