During the time of JCPOA (original, between Iran and the P5+1) inspectors had access to where they wanted to go (sometimes with friction, sure) and were able to place tamper resistant / tamper revealing instrumentation, air filters, and spectrometers - effectively creating a data record that could place a stochastic cap on {enrichment level, volume}.

After Trump ripped up that agreement during his first term, withdrawing from the pact in 2018, that was no longer the case - leading to your linked 2023 statement.