The rules didnt change for you, but they shouldve been changed for Orient. Theres a pandemic causing havoc to the football calendar and yet they get thrown out because there's no room. Teams will regularly play every 48 hours later this season to get all those postponed games in. Theres always room.
How? The next match in the League cup was 5 days later! How do you think they could have got a game organised, played, and then the next match played in 5 days if the Leyton Orient squad all had covid, and couldn't play for 2-3 of those days?
For reference, the game was scheduled originally for the 22nd Sept, and on the 25th Orient were still down with covid, with the 29th/30th Sept being the next round. They were playing Spurs, who then had matches:
in the League on the 4th/18th/25th Oct, 1st/8th/21st/29th Nov, 6th/13th/16th/20th/27th, then the Europa League started and they had matches on the 22nd/29th Oct, 5th/26th Nov, 3rd Dec/10th Dec. The only gaps? the 2 week period between 5th and 17th Oct, and 9th and 20th Nov - both of which were international breaks (England for instance played three times in both gaps) - so not feasible to schedule. Leyton Orient also had fixtures on every single Tues in Oct, Nov and Dec, other than the 21st Nov (middle of the international break, so Spurs couldn't play), 1st December (Spurs had a match in Austria 2 days later) - so even if you argued you could force Spurs to play a Sun EPL game, then the Tues for the FL Cup, then on Thurs in the EL, then Sun again in the league, all those Tuesdays are already taken up by Orient already having fixtures on them days.
So if you had postponed it, the next week where a day would be free without BOTH Spurs playing 2 games in 2 days (Weds and Thurs), AND Orient playing two games in 2 days (Tues and Weds), was the 1st December. Thats 2 months after the NEXT round of the League Cup would have been completed. But sure, let's pretend that is even vaguely possible. Then you have to schedule a Spurs vs Chelsea (assuming they play each other) - in Dec, both teams were playing twice a week *ALREADY*, but let's do that. How is this fair to the integrity of the competition that you'd have 1 match in the round taking place months and months after all the others have been completed, AND to do this you are forcing Spurs to play 4 times in a week TWICE. I know you are an Arsenal fan, but surely you can see this would be taking the utter piss.