Playing by intention. Is this common?

I thought this was common but maybe not?

Playing in a competitive league. I play Orks, opponent custodes. I set up my Gretchin in my deployment zone in order to prevent any deep strikes from behind; models placed 9 inches away from my battlefield edges/corners. I informed my opponent of this.

Later in the game they then deep strike their Callidus Assassin into my deployment zone corner saying that they had enough space to place their model there. Naturally I was pretty bummed, especially since I had mentioned I was setting up my units to prevent exactly this from happening.

Also, we weren't playing on a game matt and just keeping track of battlefield edges with dice markers. So I thought it was understandable that I might have been off with my measurements by a few cm. I really feel I was in the right, but ultimately didn't feel I could argue with the fact that yes, there did appear to be space for his model.

Thoughts? Should I just assume competitive play is not the place for play by intention?

EDIT: Thanks for all the comments everyone. I did not get verbal confirmation, so that's my error. Still butthurt but I do understand playing by intention more now. Thanks again everyone 🙏🏽