They will argue it's not that though and saying they just want to make Modlin Airport more viable and that Ryanair's rates are not sustainable and to reduce the overcrowding at Chopin Airport and they are just trying to be fair to all carriers and to take the pressure off Chopin Airport which PPL are apparently going to close during certain hours in the future and want Modlin to take some of the strain.
The issue of capacity at Chopin Airport is a curious one, about 10 years ago they built a new terminal, T2. Then they closed T1 and refuribshed it and everything was then defined as the same terminal but parts of the old T1 are unused now. Also we've often heard the "no space" argument in the last 3-4 years when some airlines have talked about expanding but others have been able to expand without issue.
Worth noting is that there were incentives for new routes to serve Chopin Airport that were quite loose before Ryanair went back there, they were tightened up a little after and a lot of pre-conditions were put in that wouldn't effect many airlines much but would effect Ryanair. There was also talk of incentives being offered only for airlines that useed air bridges but I don't know if this actually happened in the end.