Not saying I agree that being closed source is in the public good, although one could certainly argue that accelerating the efforts of bad actors to catch up would not be a positive.
If that was true, then they shouldn't have started off like that to begin with. You can't have it both ways. Either you are pursuing your goal to be open (as the name implies) or the way you set yourself up was ill-suited all along.
There's nothing wrong with changing your opinion based on fresh information.
What goes unsaid, perhaps, is that back then (before the transformer had even been invented, before AlphaGo, what people might have imagined AGI to look like (some kind of sterile super-intelligence) was very different from the LLM-based "AGI" that eventually emerged.
So, what changed, what was the fresh information that warranted a change of opinion that open source was not the safest approach?
I'd say a few things.
1) As it turned out, OpenAI themselves were the first to develop a fledgling AGI, so they were not in the role they envisaged of open sourcing something to counteract an evil closed source competitor.
2) The LLM-based form of AGI that OpenAI developed was really not what anyone imagined it would be. The danger of what OpenAI developed, so far, isn't some doomsday "AI takes over the world" scenario, but rather that it's inherently a super-toxic chatbot (did you see OpenAI's examples of how it was before RLHF ?!) that is potentially disruptive and negative to society because of what it is rather than because of it's intelligence. The danger (and remedy) is not, so far, what OpenAI originally thought it would be.
3) OpenAI have been quite open about this in the past: Musk leaving, being their major source of funds, forced OpenAI to make changes in how they were funded. At the same time as this was happening (around GPT 2.0), it was becoming evident how extraordinarily expensive this unanticipated path to AGI was going to be to continue developing (Altman has indicated a cost of $100M+ to train GPT-3 - maybe including hardware). They were no longer looking for a benefactor like Musk willing/able to donate a few $10's of millions, but needed a partner able to put billions into the effort, which necessitated an investor expecting a return on investment, and hence the corporate structure change to accommodate that.