We could start with taking anything said by Ed Lyman with a huge grain of salt, considering he's made an entire career out of concern-trolling anything related to nuclear power.
In this case, his argument is basically that HALEU is a proliferation risk because given access to enrichment facilities, it's more effort to produce weapons-usable HEU starting from HALEU than from LEU or natural uranium. Well, duh! As he himself admits in that letter, it's only a modest factor of three difference for HALEU vs LEU. From a non-proliferation perspective, the critical thing is the access to enrichment technology in the first place!
If one wants to make a case against HALEU, compare the economics vs. LEU.
In this case, his argument is basically that HALEU is a proliferation risk because given access to enrichment facilities, it's more effort to produce weapons-usable HEU starting from HALEU than from LEU or natural uranium. Well, duh! As he himself admits in that letter, it's only a modest factor of three difference for HALEU vs LEU. From a non-proliferation perspective, the critical thing is the access to enrichment technology in the first place!
If one wants to make a case against HALEU, compare the economics vs. LEU.