with a Sit-In Kayak (SINK:-)
close to shore here on Lake Ontario
water temperature: 15°C (which is 59°F I believe)

Xiaolu, the YouTube video picture and link aren't appearing in your post.  I had to hit "quote" in order to see the link, and then copied it and pasted it into my internet browser to view the video. I tried to include the link in this reply, but it also didn't appear in this reply.

With no drysuit and no PFD in October, Commando Li could have gotten into serious trouble if (a) he had accidently tipped the kayak over even one more time, because it looks to me like he was getting cold, and barely made it back into his kayak, (b) he had gotten more water into the kayak when it flipped over and had completely swamped it, because the recreational kayak might not have enough flotation to support him when completely full of water, or (c) there was any wind or waves, which could have swamped his kayak when he was trying to climb back into it.

When people are thinking about buying a recreational kayak and ask me for advice, I recommend that they get a sit-on-top kayak, not a sit-inside kayak, because if they tip over when paddling a sit-inside kayak on a lake when they are any distance from shore, they could get in serious trouble unless they are accompanied by several friends in kayaks who know how to drain the kayak and get them back into it.  If you tip over a sit-on-top kayak, all you have to do is flip it back upright and climb back on top of it.

Seriously?  From your earlier post about dry pants/dry tops, it sounded to me like you haven't become proficient in self-recovery, and I'd think that if you'd both ended up in the water, your day was going to get mighty interesting mighty fast.  Especially if "Commando Li" had been in serious trouble or had panicked.

Even practice sessions need some common-sense planning.

My buddy ("Commando Li") was with someone else in the video, not with me. I wasn't present.