won’t they feel it’s unfair and they are being treated differently
Not at all, assuming you treat them all the same, and expect everyone to obey the rules, be respectful etc regardless of whether they are blood or step. It also assumes that what you are expecting of your step kids is no different to what you expect of your own, and is not unreasonable.
The difference with your step kids is that you don't have the automatic unconditional love you have for your own kids, and they are often struggling to get their heads around being torn between loyalty to their mother, and liking the step mother. Your step kids also don't have the default setting of loyalty and obedience to you that they do to their parents.
The other difference is that as a step parent you cannot effectively discipline your step kids if your partner (their Dad) isn't backing you 100% AND dishing out the discipline too. That's where the disengaging comes in, because it forces your partner to step up, and it makes your step kids realise that you are not a monster, you are person who deserves respect and politeness like anyone else.
The problem with a lot of relationships is that the guys tend to bury their heads in the sand when problems arise and leave their partners, the step mum, to do all the hard work. That leads to hurt and sometimes the total break down of relationships. Of course they freak out when they can't pass the buck any more - wouldn't you?
Disengaging is about acknowledging that there may be different styles in parenting, but BOTH partners have to parent. It's about step parents standing up and saying that yes you care, and yes you will be there for them, but you are not prepared to be a scape goat, or a door mat. It's about saying that you have the right to be treated with respect in your own home and not with disdain. It's about saying you want to be part of the good times as well as the bad times and that you want your partner to be an equal partner in the parenting relationship, not another child/problem to be dealt with. It's about saying you'll accept the blame for cr*ap that's your fault, but you won't take on board other people's.
Lastly it's about the kids learning that they don't have to like you, or you them, but you do have to live together, and that means treating one another with respect. They wouldn't treat their teacher or some random person in the street like that (you hope!), so why should they think it's acceptable to treat you that way? When they grow up and go to work/out flatting they will have to deal with people they don't necessarily like. Doesn't mean they can be rude to them (well ok, some people never learn that and are just mean rotten people all their lives but you want to avoid them coming from YOUR house if you can!) 