It’s a serious proposal, but not as a universal conscription. It’s intended to only call everyone in for the health check and use that as a way to get young people interested in the army.
There are different models floating around, the most serious being that everyone (including women) gets called in and you basically choose between civil service and army. The civilian side can ramp up slots rather quickly, the army doesn’t. So the army probably will ramp up over several years.
Also, I wouldn’t call 100 billion € a “modest increase”.