Short answer, yes, the North Korean military was able to escape because of the taking of Seoul.

What one has to realize is that taking a city involves neighborhood by neighborhood, street by street, house by house close-quarters combat. It is a slow, dangerous, and arduous process. MacArthur had promised Rhee that he would liberate Seoul and so he pushed his commanders to do that which bogged them down in capturing what is a very unnecessary target.

Korea has two or three good routes for heading north/south - through Kaeseong, through Cheorwon, and along the east coast (and that one is pretty mountainous). Seoul was being defended by an under-strength division with the bulk of the forces south of the city. Now remember, in 1950, Seoul was situated ENTIRELY on the north side of the Han river with the settlements on the south side of the river not a part of Seoul. Incheon is south of the river, as is Gimpo Airport. Thus, the US/UN forces took Incheon, took Gimpo, and then ran into North Korean forces at places like Yeongdeungpo and Noryangjin, currently districts of Seoul but at the time, little villages and train stations. They also built bridges across the river, west of Seoul, near Goyang, north of Gimpo airfield.

While all of this was going on, the North Koreans down south were retreating north via the Cheorwon and east coast routes with NO US/UN forces there to stop them.

If the MacArthur had pushed his troops to head east and bypass Seoul, they would have taken back the south bank of the Han river, been able to bridge the river east of the city through Hannam or Yeoju, and blocked the retreating North Korean military at Wonju. Instead, the bulk of the North Korean force was able to bypass Seoul and escape north as the US/UN forces were busy retaking Seoul.

So yes, MacArthur and his commanders allowed many of the North Koreans to escape, but it wasn't due to cautiousness as much as it was to retaking Seoul instead of heading towards Wonju.