Any time someone leaves a company, they lose access to the private company/government network, so they lose access to the private email also, even if the email account is not deleted. They don't use public email like gmail, hotmail, etc. Access to the private network is taken away on the last day of employment, even if one resigns. This is standard procedure in all companies, including government agencies. But you are correct it was not smart to fire them in the first place if they are actually needed.
I worked at one company where 40% of our department got laid off, but right after that 3 other people resigned (including me) so they tried to hire back one of the people who got laid off, but he refused to come back because his severance package was about 8 months pay.
They can still find those people who got laid off, because those former employees still have to deal with government HR department for various reasons related to their severance or Cobra health benefits, or whatever. But it may just take a little longer. The media hates Trump and Musk and just like to exaggerate stuff.