Someone must have heard somewhere about knitting on Motzei Shabbos and dedcided this was the halacha and told someone else.
I didn’t get that from the OP. The OP heard from somewhere that knitting was assur motzai Shabbos and asked the CR for a source.
Someone heard it was assur to eat Pizza on Motzei Shabbos and told someone else and now they think they are doing wrong by eating Pizza on Motzei Shabbos.
If a hurricane were bearing down on my city and I stayed in my beachfront house because I heard from my neighbor Bob who heard from his pal Joe that the storm was turning away from the coast and I didn’t need to worry, and I failed to check the latest information from the National Weather Service, or whether an evacuation order was given, wouldn’t I be stupid? Honestly, if halacha is important to me, why would I change what I do towards either stricture or leniency based on what someone heard from someone else who heard from someone else? Or even a non-halachic book? I have a rabbi and a brain, and if I fail to use them properly and do something wrong or unnecessary as a result, then it would be my own fault!