He pulled the same 'stop the steal' crap in 2016, but since then has done precisely nothing to prevent vote fraud. If he actually believed it was happening, don't you think he would try to do something to stop it? If it was me I'd make it the primary plank of my administration's policy, but no, nothing.
Firstly he knows it's a lie, but anyway if he did reform the electoral system he would no longer have an excuse when he lost because he would be responsible for ensuring the election was fair. The guy and his stop the steal supporters clearly have demonstrated that they have nothing but contempt for democracy or the institutions that protect it.
They don't even care enough to reform or improve the system. Where are their attempts to do so? Their only policy is to usurp power and suppress the democratic process, and that's a very familiar strategy to anyone who has lived under facism. Check out Arnold Schwarzenegger's articles and videos on this, the guy grew up in post-war Austria and knows what he's talking about.
But the existence of some such people are being used at an excuse to dismantle democracy.
Obviously most Trump supporters aren't Nazis, but they also don't really seem to mind having them around.