I just tried on my iPhone and it does not do that, there is no countdown. It will force a passcode and give you the option to call SOS, shut off your phone or show your medical id.

It's a setting (Settings > Emergency SOS). It used to be on by default and do a little siren sound before calling emergency services.

Personally, I just open the slide-to-turn-off phone screen instead (hold volume + side button for a couple seconds). Once that screen is loaded, it'll require a passcode to unlock after you cancel out.