You absolutely cannot build a fission bomb with uranium enriched to only 20% without further enrichment. I don't know who's saying that but they're full of shit. You need uranium enriched to 80% at a minimum and even that requires a very clever bomb design and exotic neutron sources to supplement the uranium. Most uranium bombs require 90-95% enrichment to work and even then resist miniaturization.
This paper isn't actually about US SMRs. It's a stalking horse for claiming that the Iranian nuclear program, which halts enrichment of uranium at 20%, is a proliferation risk in its current form rather than requiring significant re-enrichment of its stocks that would be rather easily detectable under the framework of the 2014 E3+3 deal.
Except... Iran hasn't halted uranium enrichment at 20%. Try 60%. Source: International Atomic Energy Agency report dated 26 Dec 2023. "In a Dec. 26 report, the IAEA noted that Iran is now producing approximately nine kilograms of uranium enriched to 60 percent uranium-235 per month. Iran was producing 60 percent enriched U-235 at a similar rate in early 2023, but decreased production by about two-thirds in June. (See ACT, October 2023.) Accelerating the production of uranium enriched to 60 percent U-235 is concerning because the material can be quickly enriched to weapons-grade levels or 90 percent." https://www.armscontrol.org/ac...) If you really believe Iran is only pursuing "peaceful nuclear energy solutions" I have a bridge to sell you, as well as some swamp land in Florida.
In the long run, every program becomes rococco, and then rubble. -- Alan Perlis