The actual answer is that this interesting phrase creates a double meaning, or two valid meanings at once. Collecting money is the primary meaning. However, the author uses specific wording that is vague enough that collecting actually can work with both meanings. Mr. Dursley sees people who are oddly-dressed, and naturally concludes that they are collecting for some purpose but, also, at the same time, speculates that the purpose is an intention to collect money. The sort of unconventional language of the author, not specifically saying the more common collecting money for something, was probably specifically done to create this double meaning.
A double meaning is a literary device that one sees quite often especially in works of fiction from better writers. The wording could also have been: they were obviously collecting to be collecting for something or it could have been they were obviously collecting for something to be collected. Neither of these alterations changes the meaning of what was written at all. But I can only make these alterations without changing the meaning of the text because there is a double meaning of collecting as I described.
In this case you need to provide the surrounding text for context. I found:
As he sat in the usual morning traffic jam, he couldn't help noticing
that there seemed to be a lot of strangely dressed people about.
People in cloaks. Mr. Dursley couldn't bear people who dressed in
funny clothes -- the getups you saw on young people! He supposed this
was some stupid new fashion. He drummed his fingers on the steering
wheel and his eyes fell on a huddle of these weirdos standing quite
close by. They were whispering excitedly together. Mr. Dursley was
enraged to see that a couple of them weren't young at all; why, that
man had to be older than he was, and wearing an emerald-green cloak!
The nerve of him! But then it struck Mr. Dursley that this was
probably some silly stunt -- these people were obviously collecting
for something... yes, that would be it.
Given the surrounding text so far there are a lot of people who have similar odd-looking clothes on and therefore this supports collecting in its primary meaning when there is no object - that is to gather together; assemble. So far, there is not any active collection of money, clothes, etc. taking place. The people don't approach him as he sits in his car and ask for contributions - they are just people that Mr. Dursley notices as an oddly-dressed group. Thus one of your translations you found gives this meaning of collecting and without the additional textual clues it seems valid enough. However...
As @Adam pointed out there is an additional important clue two paragraphs down that I found here (with @Adam's help). The text two paragraphs down states:
He'd forgotten all about the people in coats until he passed a group
of them next to the baker's. He eyed them angrily as he passed. He
didn't know why, but they made him uneasy. This bunch were whispering
excitedly, too, and he couldn't see a single collecting tin.
This additional clue makes it clear that Mr. Dursley initially believed that they were gathered together for the purpose of collecting money.
What is interesting is that therefore this is really a double meaning of collecting. Mr. Dursley clearly understands that there are people gathering together or collecting, one definition, and also thinks that the purpose of their gathering together is to collect money, or collecting. Collecting money is the primary meaning. However, the author uses specific wording that is vague enough that collecting actually can work with both meanings.
