Well, saying
“On our watch, for the first time in 10 years, seniors are going to get the biggest increase in their Social Security checks they’ve gotten.”
Isn't quite lying, but it is playing silly games with statistics (and 'stretching the truth' would, conversely, be an insufficient term to describe the degree of dishonesty), because it's only referring to dollar increases, not real-terms ones.
I do find it hard to understand how Biden could say that with a straight face because the flaw in it is so glaringly obvious he surely has to know it's a dishonest comment. I'd have more respect for him had he looked embarrassed when saying it.
But ignoring inflation when talking about budget increases is something that many politicians have done at various times, it's a common trick, the Tories do it regularly when talking about NHS spending. e.g.
David Cameron and Jeremy Hunt have been ordered to stop claiming that NHS spending has increased after the official statistics watchdog found health funds had fallen.
www.telegraph.co.uk
(Actually that case was even worse, becuase he specifically used the phrase "real terms")