Is it always OK to ask for a copy of the lecturer's slides?
It is always appropriate to ask. But some people are unwilling to share their slides for a variety of reasons (none of which I think are particularly good, for basically the reasons you mention in your quote).
So you may or may not get the slides, but it is certainly ok to request them!
In my opinion it is always ok to ask for something. One thing to keep in mind is the question whether the lecture was public. In Austria most lectures from universities are public so anyone can join and listen/participate (only lectures, no seminars).
A reason for you not getting the slides may be that the professor was just busy and forgot to reply to your mail.
Another point to keep in mind is: Do you need all slides? Is there something the interests you particularly and you know the number of the slides. Then I'd suggest to ask for this slide rather than the full set of slides. I've seen situations where lectures have just been copied by a lecturer. So some lecturers might be cautious to send their slides.
The answer depends on what you mean by "OK". If by that you mean "not inappropriate", then sure. It isn't some kind of taboo or insult. But if you mean when is asking even a bit too much, well if they didn't respond apparently it was.
Every email request that requires the other person to perform some new task runs a high risk of crossing that line and not getting a response. I.e. requests where they have to go look something up, or dig out some document to attach, etc. Sometimes people mean to but then forget, but often they just think "nah" and move to the next email.
This is magnified greatly when the person receiving the request is in a position that gets many such requests every day. And moreso in a academic jobs where they are always fighting to get out from their todo list to make time for "real work" like writing and doing research.
Finally, if the person requesting the information has absolutely no relationship with the other person, the odds go down another order of magnitude. In such cases the person is basically a saint if they consistently respond to everyone. Yes I know such people too.
Conversely, I'd note that when you put someone on the spot in person and ask if they will do you some favor such as this, it makes them uncomfortable to refuse directly even if they want to. So they may give you an agreement they didn't mean. Indeed in some cultures it is supposedly impolite to decline a request directly, which is often seen as too blunt. So they will agree (or seem to agree) in a face-saving not-really-agreeing way that is hard for outsiders to read as a refusal.