Pure fusion bombs are very destabilizing, yes. The current nuclear arms control regime depends on uranium enrichment/plutonium production being heavy industry, requiring large facilities with unusual equipment.

A research program for EPFCG pure fusion weapons could be quite small, only requiring a few hundred people and little equipment that couldn't be manufactured indigenously. Test explosions could be done at the kilogram scale, producing no radiation detectable from orbit or seismic effects, then easily scaled to kiloton yields.