Ok, well taking this at face value.
there can be a lot of reasons why a teacher might ask a parent or guardian not to teach stuff outside school.
the most obvious is that the parent might be teaching it wrong. So for example with phonics, children are taught the sound a letter makes to start with not what it is called. If a parent has taught them that m is called m then when they get to school and are taught it makes the sound mmmm they can get confused.
sometimes parents teach things that are just not factually correct - most teachers have had the experience of a child saying “my mum says” and it’s something completely wrong. Normally we just sidestep the issue unless it’s really something that needs to be adressed “my mum says the moon landings never happened and NASA made it up” type stuff.
in my subject maths, there are lots of methods of doing calculations. In ks2 sats only some methods get you marks. The schools teach those methods. In other countries, different methods are used.
for example, this is what Japanese multiplication looks like. https://whizz.com/en-us/blog/its-not-magic-its-math-how-the-japanese-multiplication-method-works/
if a kid is taught Japanese multiplication at home and then the standard English method at school they are going to get very confused.
most schools run information evenings for this very reason, and try to explain to parents for example how they teach phonics, so parents can work with the school and not confuse the crap out of the child.