It’s part of the addiction hun, my husband had 6 months clean last year but I’d still catch him lying about little things. I think it becomes second nature to them.
I read a post on a different site from a ex addict where she said when you’re in active addiction your lies don’t feel like lies, you see others as stopping you from having fun. Apparently it’s not denial as denial is accepting there’s a problem but downplaying it. Delusion is a better description, they are delusional in their beliefs.
This link here explains it perfectly
https://www.addictioncampuses.com/blog/5-lies-i-told-in-active-addiction/