Batteries degrade over time. Especially when they are left in an uncharged state. They start to sulfide and cannot hold charges as well. Because of this, they may trick tenders into being fully charged, when in fact they are not.
You probably could jump start the bike or bump start the bike after letting it sit on the tender, but in all likelihood if that battery is more than 2 years old, it's shot. Get a new one and save yourself getting stranded somewhere.
Also, do not jumpstart a bike like you jumpstart a car. Research it. You can at the very least blow a fuse or cook a wiring harness and in worst case scenario, burn your bike to ground. A lot of people don't seem to understand there's a certain way to put jumper cables on and that bike and car batteries are not the same.