Sounds like a scare story.

It's an incredibly unlikely situation where the two would be
simultaneously disconnected, and then reassembled so wrongly that the
incoming gas supply does a u-turn into the rising water main - and
then the two get turned on simultaneously (without noticing that
turning on the first hadn't produced the expected effect) so that
mains pressure water is driven back through the gas supply -
presumably back through the gas meter as well.

Does anyone really turn on one supply, then immediately turn on the
next, before checking the first for leaks/purging etc?

They do *have* water and gas in Poland - so there's no reason why a
Polish plumber is any more likely to make such a horrendous mistake
than a British one.