As already pointed out, this is just not true. If you are denied boarding, it doesn't matter why, as long as you have all required docs and don't pose a health or security risk.

BA learned this the hard way when they denied me and countless other passengers boarding at ORY once because the flight we were on was delayed by an hour and they didn't want us waiting in LHR.

They tried to wiggle their way out of it, but in the end had to pony up the cash.