Agree agree agree. It drives me mad when people slag her off for not being a mum. It's like that dreadful Calpol ad from a few years ago - "if you've got kids then you'll understand". Implying that people who haven't got kids can't comprehend anything to do with parenting is really insulting to their intelligence IMHO. Like you can only be a sports journalist if you've been a sportsman, or an oncologist if you've had cancer.
She's perfectly well placed to advise on ways of caring for children having done it for years. The lack of genetic relationship is neither here nor there ( if it were, presumably adoptive parents couldn't have any knowledge on the subject either?)
Also in my opinion people tend to project hearsay and personal insecurities onto her. I have never read a single statement in her books along the lines that crying children should be ignored, that you should keep babies awake until she says it's OK for them to sleep or that failing to do what she says makes you a bad mother. She's actually pretty careful to say the opposite but it doesn't stop people seeing what they want to see.
I should add I'm not GF nor an acolyte but I do follow her routines and find they work very well. They're not perfect and I have adapted them to suit me, but then I didn't expect motherhood to be a situation where I delegate all problem-solving to someone else who will have all the answers.
OP, if you are attracted to the idea of a routine then I would recommend her approach. Routines aren't everyone's cup of tea, and you should read her with a little pinch of salt at times, but I reckon she has a method that works if you follow the basic principle of consistency in all things as far as possible, and learn to recognise when your babies are tired and need to switch off. It may be a bit much to expect them to fit into it until they are a little older (ours were about 6 weeks) and I found it helpful to introduce it in stages rather than all at once. Good luck.