Remember advice does genuinely change because things change...
If you're obsessive about sterilization and anti-bacterial wipes endlessly, then "the virgin gut" (presumably meaning not colonized by the bacteria that have to be there before you can eat non milk foods) can last a long time, however if you're less obsessive about such things and the bacteria transfer happens earlier. And when you introduce potential allergens into the gut without the bacteria there - allergy is perhaps more likely.
So if you're very anti-bacteria/hygiene obessive delaying allergens is a possibly a good thing. That doesn't mean it's always relevant. It's similar to the advice on salt for adults, if you have high blood pressure, reducing it helps, if you don't, salt appears irrelevant. Unfortunately though simple mass health messages - particularly once percolated through groups and websites etc. etc. can't go into all the details so the advice becomes a catch all.
Without high blood pressure salt is probably irrelevant, with it reducing is probably good - so the advice can apply to all and it's a simpler message.
So the advice on all sorts of things can change because the wider society has changed rather than the science or understanding behind it. People desire to get simple messages, it's a shame, but it's almost certainly better than no advice.