Boeing is having a rough time of it right now, with parts falling off its planes left, right and center. Just last week, a wheel came loose and smashed through a car, and earlier this year the door from a 737 Max aircraft broke off mid-flight. That mid-air disaster sparked an audit from the Federal Aviation Administration, which has gone far from well.
There’s nothing wrong with using soapy water to spotcheck airtight seals.
Using soap as a lubricant is a bad idea in general.
For the key cards, yeah they should be using feeler gauges of specific, certified thickness.
Soap is bad if you need to keep something lubricated but it works pretty well is you just need to lube something up a bit for installation because you can just hose it off afterwords. As far as the keycards go, that depends on the tolerance. If that tolerance is generous enough then an actual feeler gauge is overkill. A pack of keycards is dirt cheap and are all going to be about the same thickness. When one gets busted or worn then you can just grab a new one from the pack. Verses having a production guy break a feeler gauge and start guestamating because they don’t want to tell the boss they broke the third one this week.