Not having those URLS defined doesnt mean clients cant connect, its just the name autodiscover passes to the client.
Internally, domain-joined clients will be able to connect to Exchange.
If you remove that internalHostName and than do an Test Email AutoConfiguration: https://kb.intermedia.net/article/2150, you can see what is returned by Autodiscover
External Clients connect to the ExternalHostName, but if the internalhostname is resolvable externally, they can still connect as well.