You're correct. Since March, Biden has done minimal campaigning, when he has needed to act presidential and make statements he has appeared in person (and has often been masked and social distancing to lead by example), but primarily he's sat back and let Trump "proceed" as Obama did with Romney in their 2012 debate, and let him make a mistake. Except Trump's been in one combo-mistake for six months.
All Biden's had to do is wait, and then put out social media statements like, "I can't believe I have to say this, but don't drink bleach."
And as his approval rating has trended up about 5-10% in that time, Trump's has gone the opposite direction. Head-to-head, Biden has a stable lead, and there's still more tell-all books coming, more former National Security experts running to the press, more people dying of Covid-19 in a terrible Trump administration response, and just more tales of corruption that keep coming out.
Biden and the Democrats are setting up quite the rope-a-dope, if Trump turns up to debate Biden. Because any mention of Ukraine or Russia by Trump is going to have the counter-punch waiting about Russian bounties. Biden hasn't spoken to that yet, and my money is he's waiting for a national audience to deliver that uppercut.