Thank you. Below is what the received email said.

I don't know any other meaning of the words: "collected Avios" are "on their way" and they "will be credited within 5 days".
I take "pending" means "not credited yet" as opposed to "subject to approval".
Had they been "subject to approval", I wouldn't have been informed they would appear on my account within 5 days.

I followed the purchase rules of the advertised offer on BA e-Store and once I received the below confirmation, I assumed I was dealing with a professional retailer.

Nothing looked "incorrect" to me (as the invitation to double-check the amount is mentioned in the received email). -

Is there any conditional or ambiguous information in the email that I am not seeing? "Collected", "are" and "will be" look pretty committed to me.

Thank you for your thoughts.

Re: Your 21,000 Avios are on the way

Hello Mr MGUK,

Thank you for shopping on the British Airways Avios eStore on 22 October 2020.

Your Avios will be with you within 5 days.
Purchase date 22 October 2020 Retailer XYZ UK Purchase amount £1,300.00 Avios collected 21,000 Avios (pending) If any of the information above looks incorrect please check the purchase conditions on the eStore, if you need further assistance please contact us using the Missing Avios form.

You can view this and your other eStore transactions by visiting your account here.

Thanks again for shopping on eStore.