I am with Leander on this. Bottle is too small to give any meaningful underwater time. Filling by hand will be a very laborious, for only a few breaths underwater. They clam “up to 10 minutes” which will be involving breath hold and c02 accumulation to get up to those 10 minutes.
Now, since you refill your lungs underwater from this pony bottle, you will have to keep in mind that you have to exhale on the ascend, or you are guaranteed a barotrauma. I have been offered a buddy air from scuba divers a few times, while I am freediving. I always refuse, because I am scared how I can forget, mix up with these two completely different disciplines and get into trouble. To toy around with scuba, you are better off getting full on PADI certification and a proper gear.
I know some spearfishers (Bill comes to mind) use spare air or similarly small tank to deal with complicated cases like tangled up gear and such, but not for actual hunting. If you want to stack up a few such pony bottles to get deeper, you will be ascending too often and too quick for any nitrogen tissue load calculator to keep you safe.
And lastly, considering the price of that korkel-snorkel , you can get certified and buy your regulator and bcd and other bits and pieces and will be still ahead financially.