First this footage is from seven years. Second it seems you still don't understand the entire how IS entered in the first place, they came in mostly without battle.
The Iraqi army divisions in the north had well over 50% Kurdish and Sunni soldiers and officers. Most of these officers and soldiers were not ready to fight for "Maliki". In the end the Kurds and Anti-government Sunnis just wanted any way to get rid of Shia control. When IS entered most of the officials knew about it, both Sunnis and Kurds agreed not to fight. The Kurdish officers took their soldiers and equipment and went to Kurdish territory, while the Sunnis simply threw their weapons and went home. The few remaining Shia soldiers fought until they received order to retreat.
This explains why IS didn't attack the Kurds in the beginning, and all the Sunni officials went on to praise the "revolution". Most of these Sunni officials were being housed in the hotels of Erbil although they were wanted by the Iraqi government.
In the end this caused a ripple effect, most of the army collapsed and retreated from the Sunni areas as local IS population started apearing in public.
But let's look at the other side. IS put all their strength to take over Shia towns and villages yet they couldn't. The town of Amirli is a great example. A small town of less than 12,000 population surrounded by IS from all sides and under siege for 4 months. Yet IS failed to enter it simply because the locals weren't going to allow them.
Another example is Beiji refinery. Largest refinery in Iraq. Supplies all the northern Sunni regions as well as the Kurdish regions. It has 15,000 employees and 500 security. When IS approached. About 400 of the Sunnis soldiers decided to leave their posts. When the mostly Shia special force unit which was based inside the refinery ordered them not to, they threatened to fight their way out. In the end the Sunni soldiers agreed to leave their weapons and leave. These 100 special forces who were mostly Shia from the south stayed under siege for 5 months, repelling wave after wave of IS attacks and killing over 400 IS members in the process.
It's basically about having the will and motive to fight. Either way the pro-IS population have exposed themselves. When the siege was broken off Amerli. Over 30 surrounding villages which housed IS were razed to the ground, and anyone speaking about secterianism can go screw himself. It's a secterian war which IS has started, and the only way to get rid of IS is to remove the population which supports them.
For the record, the persons who are leading the battles now are not these police officers, the groups leading are AAH, BADR brigade, peace brigades and an alliance of over 30 different militias which will make the IS-supporting population regret they were born.
One other thing for those who are happy with this chaos, I want to remind that it is Sunni areas which are getting destroyed, it is the Sunni population forced out of their homes, it is the Sunni cities which now lack services, it is Sunni children which are not attending school, so be as happy as you want. The Shia territories remain largely untouched by this conflict.