OP I assume you were going to take the points for your husband so completed the form with your details.
Again, just an assumption, but maybe the police confirmed there was photographic evidence after receiving your details and so you have now had to admit you weren't driving (often happens in these situations).
The letter you receive after a speeding offence isn't to confirm the registered keeper, it is addressed to the registered keeper and they have to confirm if they were the driver. So your explanation doesn't make sense.
The Section 172 form that your husband will have received shows -
Section A: fill in this section if you can confirm you were the driver
Section B: is to confirm you were not the driver, not the owner/keeper because you have sold the vehicle or it's a hire vehicle, etc.
Section C: give details of the driver as per reasons given in section B
It will be decided at court whether it remains a driving offence only, or if they will pursue you/your husband for providing false information to avoid prosecution.