I think they missed the first sentence these plants are just 1.5 weeks old
right?

If so you should be fine they will just think it is springtime and days are getting longer. It may be better to just go to 18/6 and get them growing faster now instead of spread out.

This is only an observant opinion. I have never actually done it so if someone has done it or knows for sure what happens feel free to correct this.