I think there is some validity to the question.
The poll is not that great.
In the 1960s you saw armed demonstrations. You saw blacks rioting around the nation, many disarmed. You had the cold war, and the soviets could clearly take advantage of the situation and arm many groups from the anti-war protesters to the militant blacks. Not just blacks but many minorities, Cesar Chavez was doing his strikes and boycotts at the time.
The hippie element was ideologically compatible with much of the socialist/communist doctrine. Take from your productive neighbors and "share" to support those without wealth (or creating any.) Other large segments of the population were simply joining the various political hatred groups as a result of the draft and Vietnam War.
There was many elements of society with separate ideologies that could have very quickly formed an insurgency that could hide and gain support from a significant percentage of the population. If the Soviets helped such an insurgency form they could have accomplished the exact thing the Americans accomplished doing the very same thing in Afghanistan in the 80's. The toppling of the Soviet Union through the US supported Afghanistan insurgency.
You may have seen the toppling of the USA through the Soviet supported insurgency.
The US government certainly saw the threat, and even used illegal methods to combat it.
The Black panthers legally marching while armed on a capital, resulted in immediate passage of state level laws (not just California, but at least Oregon, and Washington state as well) prohibiting the ability to do so in the future.
Before that Open Carry was legal.
Consider the California "Assault Weapon" ban of the mid 90s proceeded, inspired, and was directly responsible for the national level ban. I think it would be quite shortsighted to imagine the perceived need to legally prohibit armed assembly in light of the Black Panthers and similar groups, did not significantly influence the creation and passage of the GCA of 1968.
The recent assassinations would have just added momentum to arms restriction talks already going on amongst the legislators.
The government wanted to control the RKBA and legal possession of arms to minimize threats to itself.
In fact programs like COINTELPRO by the FBI were actively trying to destroy numerous groups like the Black Panthers, or virtually any group that could pose any threat whatsoever of any race or ideology. Turning them against each other, using prosecutions and legislation, working to destroy them from within and without. Even internal spies to create drama between members and encourage internal assassinations.
The GCA would greatly help such programs in targeting any perceived armed threats as most movement leaders had previously been arrested as part of the movement, and were often felons as a result.
The GCA of 1968 established many things that enabled direct government oversight of arms, prohibited felons (so anyone of any group only needed to be prosecuted once to never legally be able to carry again) amongst other things that would limit the capability of any large group to be armed.
So it gave direct federal government power to deny anyone viewed as a threat the RKBA.
A very powerful tool in combating any potential insurgency or uprising.
So I most certainly would not consider the GCA solely a result of the assassinations. In fact I would consider those to merely be acts towards the end which enabled already long discussed and desired restrictions to be implemented with little opposition.
Much like the antis pounce after any gun related massacre in an attempt to pass what they already desire in modern times.
The guy armed with a handgun walking around with a sign that was essentially quoting a call to assassinate tyrants by a founding father was a theat. A threat arguably intended to be protected by the Constitution, but a threat nonetheless. Such threats make the targets of those threats scared. Legislators scared of physical force tend to arm themselves better (with better or more capable security details, body guards, and larger budgets and freedoms for such forces), and try to to disarm those scaring them.
Which results in a reduction of gun rights for everyone else.
The Phoenix display may not have had the same threat as in the New Hampshire incident, but it comes shortly after, likely inspired by that very incident.