There are several opinions that it actually refers to the forecourt:
Rashi:
אל פתח אהל מועד [HE SHALL BRING IT NEAR] UNTO THE ENTRANCE OF THE APPOINTED TENT — He himself must attend to bringing it right up to the forecourt (where it is handed over to the priests).
Daat Zkenim:
אל פתח אהל מועד יקריב אותו, “he shall bring it to the entrance of the Tabernacle.” This line is necessary so that we understand that it is not good enough for someone wishing to offer an animal sacrifice to say to the priest: “here is the animal, take it and offer it on my behalf.” If he did so, it would not be considered good manners vis a vis Heaven. The owner of the beast in question had to bring it personally all the way to the entrance of the Tabernacle [beyond which he was not allowed to go. Ed. (emphasis mine)
Chizkuni:
אל פתח אהל מועד, at the entrance of the Tent of Meeting;” the Torah stresses this as the donor is not allowed to invite the officiating priest to come to his home and slaughter the beast in question in the donor’s or the priest’s yard, and perform the rituals connected with it. This would be disrespectful to Hashem . Imagine a donor of a gift to a mortal king inviting the King to come and pick it up at the donor’s home!”
Siftei Chakhamim (commentary on Rashi):
Until the court. The word יקריבנו is from the root of קורבה, which means to bring close. Rashi’s proof is [that otherwise,] why does it say, “[He shall bring an unblemished male] to the entrance of the Tent of Meeting”? It should write: “An unblemished male he shall bring to be accepted favorably before Hashem,” and no more.
Tur HaArokh:
אל פתח אהל מועד, “to the entrance of the Tent of Meeting.” The Torah needs to tell us that the donor cannot ask the priest to come to his house and pick up the animal he designated as a sacrificial offering. The owner or his messenger has to bring it.
Bekhor Shor:
אם עלה קרבנו דבר העולה כלו לאשים קרי עולה. אל פתח אהל מועד יקריב אותו. שלא יאמר לכהן: קח בהמה בביתי ועשה ממנה קרבן, דאין זה כבוד של מעלה, אלא הוא עצמו יקריב וידדה בהמתו לפני המלך:
To the door (entrance) of the Tabernacle he shall bring it
And, there are opinions that especially explain the deeper meaning behind that it cannot be the entrance to the Holy Place, as the Chizkuni explains:
אל פתח אהל מועד, at the entrance of the Tent of Meeting;” the Torah stresses this as the donor is not allowed to invite the officiating priest to come to his home and slaughter the beast in question in the donor’s or the priest’s yard, and perform the rituals connected with it. This would be disrespectful to Hashem . Imagine a donor of a gift to a mortal king inviting the King to come and pick it up at the donor’s home!”
And the Daat Zkenim shares this idea:
אל פתח אהל מועד יקריב אותו, “he shall bring it to the entrance of the Tabernacle.” This line is necessary so that we understand that it is not good enough for someone wishing to offer an animal sacrifice to say to the priest: “here is the animal, take it and offer it on my behalf.” If he did so, it would not be considered good manners vis a vis Heaven. The owner of the beast in question had to bring it personally all the way to the entrance of the Tabernacle [beyond which he was not allowed to go. Ed.
