But no it doesn't. Until your service is started, your service is NOT actually booted. That's what I said.
You are not paying per-second for the VM. The VM itself adds zero value to you. It's the service that's running (or in this case, not) that you're paying for.
Who cares how long it takes before systemd calls listen()? Nobody derives value from that. You're not paying for that. You're paying for the SERVICE to be ready. And if you're not, then why are you even spinning up a VM, if it's not going to run a service?
Starting services in parallel will reduce overall service start up time as well, even if services are dependent on each other, because services often do work before they connect to a dependent service. Without socket activation that is a race condition.
With per second billing, fast boot times save money and enable lower fixed capacity, further lowering cost.