Biden is 100% correct on this. Here he accuses Saudi Arabia, Qatar, UAE, and Turkey of flooding money and guns into the Syrian rebels, pretty much to anyone who would fight Assad. Their attitude was that if Al-Nusra, ISIS and Al Qaeda were fighting Assad, then it was worth it to fund and arm them. We have the Qatari Defense Minister on record saying just that.
The campaign to fund and arm the Syrian rebels was being run by the US, NATO and its allies Turkey, Qatar, Saudi Arabia and Qatar out of the US Embassy in Ankara, Turkey. The best evidence seems to indicate that the US only wanted to fund and arm what they called the moderate rebels in the Free Syrian Army and the US opposed arming Al-Nusra, Al Qaeda, ISIS and the more radical Islamists.
However it was well known that many of the guns and much of the cash given to the FSA ended up in the hands of Islamists. Also all of the money and guns given to FSA type groups often ended up in the hands of more radical groups as FSA-type groups evolved into more radical groups.
The notion that the US directly armed and funded Al-Nusra, Al Qaeda and ISIS and/or other radical Islamists is weak and lacks good supporting evidence at the moment. However, the US and its allies certainly fueled the fire that has turned Syria into the charnel house that it is.
