It's very relevant, because, unless exempt, if you forgot to have it with you, it's the same as having forgotten your wallet - you don't go in the shop, or you leave as soon as you realise your error.
It seems very clear that he didn't have an exemption from the post, and therefore as soon as someone pointed it out he should have dropped his goods off, turned and left the shop immediately, having realised he was committing an offence inadvertently.
I don't support the actions of someone getting abusive over masks. However, if you're not exempt, it wouldn't need to go any further than "where's your mask?", as I'm sure the confrontation started off, before turning tail with "sorry mate I forgot, thanks for pointing it out" and leaving immediately, dropping your basket before the door.
If you are accidentally committing a criminal offence - any criminal offence - and someone points it out, even if they do so impolitely, you stop committing it immediately and retreat with thanks and an apology. You don't escalate a confrontation.