Yes, you could technically use email like SMS, while the standard allows for up to five days for the message to go through that’s pretty rare, in practice it’s primarily used to send long messages from one computer to another, not a single sentence or two between phones.

In practice, it is about as secure as SMS, as both require similar levels of dedicated effort to interpret. Most of the actors with the hardware used to intercept and decrypt SMS are the same actors who can compromise a server, or outright have acess to the backdoor they paid 10 million to put in RSA. Not that they need it, as the largest email providers by far do often work with law enforcement anyway. Both SMS and email attacks are seen at about the same rate and scales, which is to say rarely outside of government agencies where both are unfortunately routine.

Signal is primarily designed and marketed to fufill the same basic role as SMS, as evident from just how much of an afterthought anything but the mobile app is, how said app copies the same format as SMS for messages, how it required an phone number to use and sync phone contacts, and how it did support SMS for quite some time. It is emently reasonable for Signal to have continued to have featured the messaging format most of the people it could talk with used.