It will be. The 20,000 National Guard troops are just for starters. Secret Service: Uniformed Division, Presidential Detail and Counter Assault Team. FBI, including Enhanced SWAT Teams and the elite Hostage Rescue Team. Washington PD, NYPD(at least 200 officers)State Police, US Coastguard, EOD units, US Marine Corp... no one is getting anywhere near Biden and Harris on Inauguration Day. Million Militia March won't even put a dent in the defences that have been set up.
Of course, while you can lock down Washington for the Inauguration, you can't do the same to every other potential target in the country. Back in 2005, the G8 was held at Gleneagles in Scotland. With the most powerful men in the world attending, the site was protected with everything from armed police to Apache Helicopters. A large contingent from the Met(London)police were sent up to help with security...and while they were up at Gleneagles, four Islamic terrorists killed nearly 50 people in suicide bombings in London.
I don't think there's much danger of anything happening at the Inauguration. But I'm far less certain there won't be an attack somewhere else in the country at the same time.