2. Majority of people do not actually post, Not you keys; not your coins, and very possibly most that post that use DEXes.
3. That phrase is mostly used when telling people not to "store" their funds on centralized exchanges. There is an obvious risk however you use a centralized exchange, but only keeping your funds there for the duration of the trade you're making and withdrawing back to a non custodian wallet reduces the risk you are exposed to. From my inference, many on the forum do this.
4. The message is the most important factor. If someone advises you to protect your wallet properly and doesn't do same for theirs, will you for that throw all security caution to the wind? I'm sure not, so apply the same logic here. If you can trade solely with DEXes and always stay in control of your funds you are protecting your funds better