To win the league you have to score goals and win games.
You can enter the Champions League if you aren’t the champions of your domestic league, no matter how long ago that was n’all. And you can win the Champions League by drawing every game 0-0 from the group stages to the final and win the knock out games on penalties.
Let’s say the hypothetical group is:
City Dortmund Valencia Atalanta
1. Atalanta P6 W4 D2 L0 F8 A4 Pts14
2. City P6 W0 D6 L0 F0 A0 Pts6
3. Dortmund P6 W0 D4 L2 F4 A6 Pts4
4. Valencia P6 W0 D4 L2 F2 A6 Pts4
Ro16
City v Barça 0-0 + 0-0, City win on penalties
1/4 final
City v Chelsea 0-0 + 0-0, City win on penalties
1/2 final
City v Liverpool 0-0 + 0-0, City win on penalties
Final
City v Atalanta 0-0, City win on penalties.
That would probably see you relegated in a league season.
Plus in the league you play every team home and away. Whereas in the CL you only play a randomly selected 7 teams (6 if you play a final against a team from your group stage group) at most out of 32 that enter.
Winning the Champions League and saying you’re the best team in Europe is like winning the FA Cup on its own and saying you’re the best team in England and Wales.
Got your point but you can also similarly make up hypothetical league results. City won 1 game and all the remaining 379 matches were drawn. City wins the Premier League.