Since you used a corporate email I think they kind of have a right to access anything with that... And you used a corporate card... But what I am trying to figure out is what you were panning to do... Did you think they would allow you to use company resources for travel after you left?
The correct thing would have been for you to cancel that when you left, then you could have changed the email and kept the account.
The employer was legally allowed to access the account, because the account was created with the company email AND the payment method for the account was a corporate card. Did you have had the account password saved in the browser, because the employer shouldn't have been able to log on without it. However, they had every right to cancel any extraneous accounts opened by former employees that were setup with company assets to lessen their exposure to risk.
@Florian478 Many Airbnb users sign in using their email address. So yes, anyone who knows that email address and password can sign into your account.
I don't quite understand- you seem to feel that your past employer is taking advantage in some way, but yet you used the company email address and the company card to book a trip that was to take place after your employment ended (maybe you didn't know you'd no longer be working there when you booked it). You didn't bother to tell him that and reimburse the company. I'd have gone in to make sure you hadn't charged trips to the company card as well.