I don’t know if i’m fully there with this advice. Some of it sounds like the writer wants you to be more monotone than you feel. To control the way you express your reactions.
Expressing your own emotional reactions to/in front of a child is surely important in their development.
That said, the writer’s key point about accidentally switching a child’s motivations for learning things through the parent’s actions is well put.
This article comes close to touching on something I’ve heard a lot, which is that praise should be based on effort rather than on outcome. Don’t say “good job”, say “wow, you must have spent a lot of time on that” or similar. I have no idea or even opinion about which is actually better, or if there even is one that’s categorically better. But I did find this article a little shallow for how it got close to that point without actually exploring it.