"The program is open to citizens of EU states or associate nations, ..."
This is incorrect - ESA is not an EU organisation.
"Please note that applications are only considered from nationals of one of the following States: Austria, Belgium, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Luxembourg, the Netherlands, Norway, Poland, Portugal, Romania, Spain, Sweden, Switzerland, the United Kingdom, Latvia and Slovenia."
The closing date was extended for applications from new ESA associate member state, Lithuania.
About ESA, the EU, GSA and Brexit
After the 2019 UK general election, the EC demanded that ESA sack all British citizens from EC funded programmes(ostensibly for reasons of security) but the UK remains a core member of ESA. The EC had their own organisation (GSA) that controlled only Galileo and EGNOS but development and procurements for these programmes have long been subcontracted to ESA (not least because GSA never had the expertise). Then the EC changed GSA to EUSPA (EU Agency for the Space Programme), broadened their scope and gave them a huge budget. Over the last year, EUSPA have been actively trying to force ESA out of Galileo and EGNOS, with the hope that they can seize ESA's staff and IP. The longer term goals of EC and EUSPA are to borg ESA, along with their assets, IP and staff.