Because there can be more than one law which applies different rules depending on a persons age. Imgur ran afoul of the rules that ICO introduced in 2021 to enforce baseline privacy requirements for children's accounts, which doesn't require absolute bulletproof verification, just a reasonable attempt. Imgur wasn't even doing the bare minimum of asking "are you a child y/n" during signup.

If it had anything to do with the OSA then Ofcom would be the ones enforcing it, not ICO.

Seeing as the investigation was by the ICO instead of OFCOM, yes, very much so. Do you have any evidence to the contrary?

The governments of the countries that dabbling into the "think of the children" laws should build their own internets for their citizens, walling them in, requiring them to "verify their age" before letting them out of their cages into the Internet.