Generally speaking (pun intended), Generals don't lie. Political candidates however do.
First, the ceasefire you mentioned wasn't a true ceasefire. Those didn't start happening until 1972/1973 at the Paris Peace Accords. What Johnson did do in 1968, was order a halt to all aerial, naval, and artillery bombardment of Vietnam five days before the election. This was a half-hearted effort to create some modicum appearance of achieved "peace" before the U.S. election, but it was extremely naive on the U.S. side, and completely illusory. The big cannons and bombing raids indeed fell silent, but the day-to-day close-in infantry ground war and dog-fighting air war stayed on at full speed. Infiltration, patrols, attacks, counterattacks and short vicious gun battles on both sides continued unabated. The difference is that the U.S. grunt on the ground was now fighting with one hand tied behind his back, as he could not request any air support or artillery support in heavy firefight situations. History records Johnson thought this bombing halt a real selling point to move towards peace talks. The prime effect of his order was to stop the US Air Force Rolling Thunder aerial bombing campaign of North Vietnam, and to muzzle the battleship USS New Jersey sitting off the coast of Vietnam which had been firing her 2,000 lb., 16-inch artillery shells 26 miles inland at various targets. The North Vietnamese were secretly overjoyed, as they were still trying to replenish and resupply their forces in the South from their heavy losses incurred during the Tet offensive earlier in the same year.
So.....as to whether the North Vietnamese used this bombing lull to sneak additional supplies down the Ho Chi Minh trail, the answer is you betcha. The North Vietnamese never stopped bringing supplies down the Ho Chi Minh trail (which was actually a network of many trails). Supplies flowed all the time, 365 days a year. Anytime the U.S. announced a lull or had an American holiday however, North Vietnam would double-down and ship even more supplies down the trail to take advantage of the lull. All the American Generals and servicemen in Vietnam knew this, and would informally mention this to many politicians on both sides around Washington DC. Nixon then was telling the truth, because he was the outsider who wanted to win the election and make the Democratic administration look bad. It's unknown what Humphrey personally believed, but in any case he followed the peace train olive-leaf party line set in place by Johnson and so publicly stated he thought Nixon irresponsible.
What finally did bring the North Vietnamese to the peace table for a true ceasefire by the way, were the enormous B-52 bombing raids launched by Nixon over Hanoi during Operation Linebacker and Linebacker II in 1972. 25 years after the war ended, the North Vietnamese admitted that of all the weapon systems we deployed against them in the war, it was only the B-52 that gave them nightmares and most terrified them. The big bombers caused huge amounts of damage and were usually so high or above the clouds that they were frequently unseen by the human eye. The bombers themselves had the ability to see through the clouds and still hit their targets. One fully loaded B-52 destroys a swath one mile long and a quarter mile wide. Several North Vietnamese generals said they would typically be randomly talking to their staff in the jungle in a tunnel or cave during the war someplace and without warning the entire jungle would vaporize into explosions, collapsing the caves and tunnels and killing hundreds of soldiers. Consequently, anytime they could use a negotiation strategy to make the B-52's stop, they took it.
