It is also misleading. It may be true that there is nothing being build right now (which may need a source) however work on additions to the main sections of the S-Bahn network is planned to start this year
We'll see if that ever happens, recently the news haven't been too encouraging, but this line in particular is doing nothing to cure the fixation of the network on the center.
and extensions to the U-Bahn network are in planning.
That may need a source. There is one new station currently being planned (Martinsried), and that's it. Everything else is still very much pie in the sky status.
Further the tram network is also constantly extended.
True, and this comes at the expense of not extending the subway, as once the tramway is extended, it's hard to argue for a subway running in parallel see also my comments about the actual existence of a ring: its eastern part is a tramway (line 16), and the western part is currently being upgraded from a bus (line 51, if memory serves correctly) to a tramway.
The reason nothing is build right now is also less related to public debt and more with politics. The city is run by the SPD and the state around it is run by the CSU. Any large scale project that involves both ends up with everyone trying to sabotage the public image of the other party and disagreements for the sake of disagreeing.
Well, and this sabotaging happens during the financing stage. I cannot disagree with there being a political side to it as you describe, but at times where Munich's debt is at a historic low even in absolute, non-inflation-adjusted terms, while it's experiencing unprecedented growth, it's silly to assume that Munich couldn't finance building subways by themselves. If only debt weren't considered bad.