I don't think you can judge it on a few meetings.
A friend recently told me that they thought a relative of theirs was treating a child badly because when the child stayed at theirs she would ask if she was allowed to come out of her room in the morning, because she wasn't at home until she'd asked. She said that clearly she'd been confined to her room and it was terrible she didn't feel she could just come out and speak to them.
Well our son has a gro clock and we tell him to stay in his room until it says it's morning. Not because we want our sleep but because he needs to try to get his! I'm sure if he stayed at my friends she'd come to the very same wrong conclusion. This could be the same with zoo. It could be very possible the child has some fantasy ideas that the SW told the parents they should correct?!? I don't know, but that's the point, no one does but the family themselves. There's a lot of stuff adoptive families have to deal with in private that impacts on what you see. For example putting an adopted child in a time out is ill advised but I bet quite a few of my friends judge my lack of time outs, at times, as poor parenting. And yet it's actually good adoptive parenting to use an alternative.
As for using Mum and Dad names, this is common practice for pre-school children and often for those at school too. If my children had struggled with it I would have maybe chosen a different method of dealing with it admittedly, but again we don't know the ins and outs of their situation, what advice they'd been given etc. Maybe that child was struggling and a SW had advised ignoring as a good way to deal with it, maybe the child had control issues and this was their control mechanism...I don't know!!! Again no-one does but the adoptive parents/SW/child.
It is also common place to not discuss a child's life with their birth family, or how they came to be adopted with family members of the adoptive family. Those discussions tend to happen between adoptive parents and their children only, for various reasons, be it child protection or simply because the adoptive parents want to let it be the child's story to tell when they are older and they won't have a choice if they go around revealing it to all when they are young.
Also, the first year or so after adopting a child is not like the first year after giving birth, it is tremendously more difficult than that. All children adopted, particularly at an age like 4, have been through a great deal of trauma and being a new parent to a pre-schooler is a shock to the system for a start. Having to not only parent, but parent a child with additional needs, is very hard. They may also have been finding their own feet at being adoptive parents and chances are that now 10 years down the line they treat the whole situation very differently (your post sounds like you don't see this child anymore)
The child will be able to track down birth parents at 18 and have access to every record, there will be plenty of info recorded for them to see. You can't brainwash a child into ignoring their prior life with birth family, you can either support them in accepting it, or choose not to support it, but that will more than likely result in a breakdown of adoptive family ties later in life. So, if none of the above ended up relating to these particular adoptive parents and they were just choosing to ignore all the advice they'd been given on merging their child's 2 lives, instead of ignoring one, it will come back to bite them on the arse.