I think they refer to the fact that, exposed as GP did, looks like there is a loophole if 2 teenagers started their relationship at 17 and 15, and once they become 18 and 16, sexting is suddenly illegal.

Best guess without looking it up, they meant to say "either" and not "both"; these are called "Romeo and Juliet laws" and are nothing new.

Huge IANAL disclaimer, but I don't think it is. It is decriminalizing some of the edge cases where reasonable, and missing the one you mention. That one isn't criminal where it wasn't previously, just unchanged, AFAICT.

I'm not claiming that the text at issue changes whether sexting in that relationship is criminalized, just that it is criminalized,† which should disqualify the law as a piece of model legislation.

It's true in a technical sense that where sexting is legal anyway, the "model" text wouldn't make it illegal, but that isn't an interesting observation, because where sexting is legal anyway, the text has no effects at all.

† Here's a citation from https://www.merriam-webster.com/dictionary/criminalize:

Many freedoms Americans take for granted -- like education, art, association, speech -- are criminalized or tightly controlled in Iran.

I suggest to you that making a change in the legal status of something is not a necessary part of the meaning of criminalize.