Isn't a house imparting impurity from inside it doing so to one entering it?
Not always. There's a Mishna later on (13:9) the describes that while you become impure upon entry, the clothes you are wearing only become impure after a certain delay.
מִי שֶׁנִּכְנַס לְבַיִת הַמְנֻגָּע וְכֵלָיו עַל כְּתֵפוֹ וְסַנְדָּלָיו וְטַבְּעוֹתָיו בְּיָדָיו, הוּא וָהֵן טְמֵאִין מִיָּד. הָיָה לָבוּשׁ בְּכֵלָיו וְסַנְדָּלָיו בְּרַגְלָיו וְטַבְּעוֹתָיו בְּיָדָיו, הוּא טָמֵא מִיָּד, וְהֵן טְהוֹרִין עַד שֶׁיִּשְׁהֶה כְדֵי אֲכִילַת פְּרָס. פַּת חִטִּין וְלֹא פַת שְׂעֹרִים, מֵסֵב וְאוֹכְלָן בְּלִפְתָּן:
If a person entered a house afflicted with a nega, carrying his clothes upon his shoulders, and his sandals and rings in his hands, both he and they become unclean immediately. If, however, he was wearing his clothes and had his sandals on his feet and his rings on his hands, he becomes unclean immediately, but they remain clean, unless he stayed as much time as is required for the eating of half a loaf of wheat bread and not of barley bread, while in a reclining posture and eating with some condiment.