I see it as more of a reminder and to pay attention. We all have ranges/spectrums at which we operate. You can say, "shouldn't you just be kind to everyone", and the answer is, "yes, of course". But then, how that kindness is shaped and applied – for example, kindness can be offering help, or waiting to be asked for help (but being prepared to help). I think it's ultimately about awareness of how you can be kind in different ways.

For those that are deemed less intelligent, that kindness can take the shape of increased patience with things that seem obvious or easy to others, whereas for folks operating at a level more similar to you, you might behave differently justifiably. This is just one example that crossed my mind in this case.

We understand that dealing with children and dealing with adults is frequently different, and we wouldn't call that patronizing, we'd call it operating at the level appropriate for them. We might tolerate something from a children, and deem acceptable for their level, while tolerating the same thing for an adult in the same context would be considered unacceptable. There are all kinds of different ways to behave differently in different context.

So, yes, "choose kindness", but I took the statement beyond its face value to mean "exert yourself to be kind" and find out what that means in the most appropriate way based on the situation.

It is not patronizing, unless any differential kindness whatsoever is patronizing. But this is clearly not the case : being overly kind to war refugees more than other immigrants is not patronizing, it's acceptance of the empirical fact that they are (with high probability) the least fortunate type of immigrants and a rational moral reaction to that fact. Similarly, being overly kind to your one friend who had just lost a family member is not patronizing. And so on. Off course you shouldn't appear to be more kind to them, because this is really just showing off\virtue signalling and it's insulting to them, but you should be more kind to them, without appearing to be so. It's difficult, but doable.

Less intelligent people lost a genetic lottery (like a lot of us with varying degrees), but the particular round they lost on is one of the worst, in practical terms. They deserve more kindness and help. No patronization in that.

This feels like an "all lives matter!" kind of response. The parent was specifically addressing the type of person who might be dismissive or unkind toward people of lower intelligence. Yes, there are people who are unkind toward all sorts of people, and that's not good either, but that's besides the point.