The difference between those earlier examples and this, is that those earlier examples were unvaccinated countries, and Cambodia will have completed its vaccine program. When you've done that, there isn't much else you can do but open up, or at least start. There shouldn't really be any need for quarantine, just testing and a vaccination requirement.
Cambodia has done well, but let's not over-egg it. Somewhere over 40% of the population is double-jabbed, but there's nervousness about the efficacy of the Chinese vaccines against the delta-variant, and talk is of a third dose of AstraZeneca. Here the good news is the low income country get access to COVAX, though it's not clear if they'll have contend with the shaky production coming out of Thailand.