I am flabbergasted at how quick people are to jump on someone, who it would seem ultimately has the best interest of the child at heart (but equally others have chosen to even name it as child abuse)!. What I read between the lines is a person who genuinely cares for that child - who wants to help provide healthy food for the child, encourage them academically, teach clear boundaries, respect, and patience. It's one of those you're damned if you do, damned if you don't situations!!! I can't help but think perhaps people are feeling defensive of their own inadequacies?
What gain is there for OP to raise these concerns? None. What benefit is there for the child - lots! She's the one trying to encourage healthy eating and respect/empathy for others - yet she is being torn down for this? It's not like she is placing unreasonable or extreme expectations for the child - like to join a shamanic cult and suck from the teet of an ox whilst walking on hot coals - she's simply highlighting it would be better for him to make healthier food choices and not get what he wants because of a tantrum. It's the fundamentals of child (human) development, nevermind 'parenting'!
It takes a whole village to raise a child
I get that the SM is not the parent - I don't believe anywhere has she inferred that she is, but what I sense is she cares about the child and wants the best for them. Ultimately, societally, we have a duty of care to children (whether it be niece/nephew, SC, friends, family etc), when that child comes into your home, and your partner chooses you to be a role model in that child's life - you have a responsibility. Whether you've only met the child once, known them 6 months or 10 years and are married to their dad. After all, behind all this is DP, enabling this relationship dynamic - DP, chose OP to be his GF. DP made a mutual decision to allow her to meet and be involved with SC. DP made a mutual decision to share a house...it's not like OP has kidnapped them and held them to ransom!
Raising children without boundaries is not healthy and has lifelong impacts. I see the OP's intentions as grounded in the right place - but perhaps not executed in the right approach and possibly clumsily worded in the OP.
Step mums - care about the child = YOU BAD
Step mums - don't care about the child = YOU BAD
