Boasting that you have more immigration than random countries like Japan is just odd.

He's not boasting; he's saying that immigration would do a lot to solve their problems; and he's correct. I hate Biden's guts but he's correct here. For context Japan is a notoriously xenophobic country and currently sits at a 2%. They're not "a random country".

In perms of the percentage of its population tho, 15% is somewhere in the middle of the pack, well behind countries like Australia, Canada, New Zealand, Switzerland etc.

I mean people deciding to come to your country isn't proportional to your population, or really related at all. It'd be like expecting China to have the same 15% as the US (for context that'd be about 250 million people). That's just not how that works.