During election season, most candidates (Trump excluded) only do something after careful consideration of the political implications. In this case, Biden knows that he can take a shot at Trump without making it too obvious. This works out great for the DNC so it can continue to showcase how bad Trump is as a candidate. Expect more of these "shot-across-the-bow" remarks as we get closer to the election.
Joe Biden recently told Baltic leaders that Donald Trump doesn't understand how NATO works. The Democrats have emphasized Hillary Clinton's experience as Secretary of State compared to Donald Trump's complete lack of foreign diplomacy experience. Anytime Joe Biden discusses the current U.S. foreign policy, he is going to use it to take a jab at Trump.
Vice President Biden reiterated America's support for NATO and Article 5; which Trump has criticized. This statement by the vice president was likely a disguised attack against Trump. The vice president is letting voters know that Trump does not have the foreign policy knowledge nor the experience to understand the role that NATO plays in the world.
Biden is defending these positions becasue they are positions that he supports and believes in. His defense of these positions has nothing to do with Trump. If Biden wanted to say something about Trump he would say it, not cloud his attack in subterfudge and talk about Trump's position on things.