You got that right.
The Soviets had backpack/suitcase nukes prepositioned in the USA since the mid-1970s
He got that wrong. The US had a roughly similar system, the Small (and Special) Atomic Demolition Munitions, from the 1950s through the 1980s. We gave up on it because it was a bad idea. The weapon was very low yield and could (barely) be carried by one strong man. Cruise missiles made these weapons obsolete.
As far as the Soviets are concerned, anybody who thinks the Sovs lost track of nuke one needs to hit some books on how the evil empire maintained its nukes. The military didn't even have control of them -- the KGB, which passed down intact to the FSB and similar services in the other Republics, had them. One reason the Soviets were always fearing that we would do a first strike, is that they might have suffered nuclear decapitation.
As far as terrorists getting such a weapon from Iran or Pakistan -- Muslims are crazy enough to try. Viz. Abdul Qadeer Khan. But -- making a small nuke is significantly harder than making a big one. To be man-portable it needs to be an extremely small nuke. Even assuming they blew off shielding (bad idea with Pu-239, even if your guy is a suicide you want him to last long enough to emplace the weapon) it's still a heavy, bulky thing.
And do not assume that the hostiles could move a radioactive weapon through the borders and through the nation with the ease they move conventional explosives.
d.o.l.
Criminal Number 18F
Many of the tactical nukes don't have the PALs you are talking about...
I wouldn't be so confident.