What is your use case here? Why would you want to send an SMS to another user who also has iMessage? Or do you mean you think that should just be a capability of any messaging service?
I don't have an iphone, but I saw a similar issue with Signal.
My wife and I use Signal to message each other and we have Signal as sms handler on our android phones. For some reason, she had to use a dumb phone with no Signal app for a couple of weeks. After a few days, she was suspicious that she is not getting some sms'es. I wanted to test if this was true, but I couldn't. Any message I sent to her number through Signal would be sent as a Signal message which wouldn't be delivered and I couldn't figure out how to send as sms. I had to change my sms app to another app, and then send using that app. Now, the new sms app couldn't read all older sms because Signal removes all sms from android sms database into Signal. So, now I use separate apps for Signal and sms.
