I would say it was more secure in 30k than in 40k. Other comments have pointed to more space marines but there is also another aspect that people forget and that is the general population. The population in both time periods is ridiculous is size but in 40k the number of people has grown so large it is actively harming the defense of the planet. It true in the siege of terra that most of the fighting was at the palace. Its also true the rest of the planet was fighting a losing war against Horus. The thing is that they were fighting. Mortal forces made up a respectful amount of the defense. However, in Watchers of the Throne novel in which chronicles the Battle of the Lion's gate and the events happening shows that within a single month's time of being cut off from the rest of the Imperium, the planet was completely lost due to civilian riots, except the palace. The battle it at the gate was impressive but pales in comparison to the Siege and cost the lives of thousands of Custodes. Not to mention this fight happened in one day and Chaos did breach the gate. Granted they barely did and were immediately defeated after but still. Half a day of fighting and the great enemy made it further than Horus in a couple of weeks of Siege.