It was stolen, not given freely. In 2016, the Israelis stole Iran's entire nuclear intelligence archive in one night. They are (and probably long have been) the world's most accomplished thieves.
And the exploits of "Demo Dick" Marcinko proved that America's defenses of its atomic weapons stockpiles is (or at least was) slipshod.
But that wasn't Israel's "first" acquisition of fissile material. France had been providing them with HEU and yellowcake (officially and surreptitiously through 3rd-party intermediaries) probably since 1954. Israelis wouldn't be Israelis if they didn't utilize that opportunity to figure a way to cut out the middleman -- France -- and go straight to the source. Which they ultimately had to do because France cut them off after the 1967 war.
To keep the supply coming, the next year Israel set up a German shell company that bought 200 tons of yellowcake from a Belgian multinational (enough for about 2800 lbs of HEU), which they smuggled into Israel.
I can't say that the 206 lbs of HEU they (possibly) stole from the US wasn't a boon, but neither was it the linchpin of their weapons program.
SRP. Decades ago