English grammar is often not very logical, and the "rules" which are used to teach it are often more guidelines, and frequently do not capture all the nuances of the grammar and usage that fluent speakers use.
Indeed the word "rule" is often misleading.
One thing I would say is that in any situation where the present continuous may be used, the present simple could also be used. Or perhaps I should say this is usually, but not always true. The continuous form emphasize the very current, often short-term nature of the situation. Let us consider the examples form your text:
- (1A) Ella stays with us quite often. The children love having her here.
- (1B) Ella’s with us at the moment. The children are loving having her here.
Sentence (1A) suggest that the children very much enjoy it every time that Ella visits. Sentence (1B) says that th3e children are enjoying her current visit, but doesn't say anything about whether they also enjoyed her previous visits. (2B) concentrates on the visit now in progress. Both sentences might well be correct, but the speaker might prefer to emphasize one aspect or the other.
One could imagine a situation where something is true currently, but hasn't been true in previous similar situations. Perhaps on some previous visits Ella was crabby or distracted, so (2B) would be true but (2A) would not. But most often the present continuous emphasizes something suggested but not stressed by the present simple.
Note that the text reads:
...we can use the present continuous with some state verbs...
Not all verbs are commonly used in the continuous construction. I can't define a good rule or even guideline for which verbs are commonly so used and which are not. In the case of "tasting" it may be because that verb form is more often used as an action verb:
I am tasting the dish.
suggesting that "tasting" is something done by a person (or animal), not a state that the food has. But it seems to work well in a sentence like:
If you add too much vinegar it leaves the sauce tasting sour.
My wife tells me that when she was growing up in Ohio in the 1950s and 60s, “the cake is tasting wonderful” would have been thoroughly natural, something she heard from adults regularly.
So which verbs are used in this construction varies by time and place, and I, at least, can define no clear rule to distinguish which can be so used. Or rather I should say "which are often so used" because others can be, and such use is grammatically valid, but may be thought odd.
