This is apparent, as the punishment for sacrificing them is stated in this verse: “And he did not bring it to the entrance of the Tent of Meeting, to sacrifice an offering to YHWH, before the Tabernacle of YHWH…that man should be cut off from among his people.” The prohibition is also explicit, as it is written: “Take heed to yourself lest you offer up your burnt offerings in every place that you see” (Deuteronomy 12:13).
מכאן ואילך – בזבחים שהקדישן בשעת היתר הבמות והקריבן בשעת איסור הבמות,
From that point onward, the verse is speaking about sacrificial animals that one consecrated during a period when there was permission to sacrifice offerings on private altars, i.e., before the Tabernacle was erected, and then he sacrificed them outside the Tabernacle during a period when the prohibition against sacrificing on private altars was in effect.
שנאמר: ״למען אשר יביאו בני ישראל את זבחיהם אשר הם זבחים״ – זבחים שהתרתי לך כבר. ״על פני השדה״ – לומר לך: הזובח בשעת איסור הבמות, מעלה עליו הכתוב כאילו הקריב על פני השדה.
This is apparent, as it is stated: “In order that the children of Israel should bring their sacrifices, which they slaughter upon the open field, that they should bring them to YHWH, to the entrance of the Tent of Meeting” (Leviticus 17:5). The phrase “their sacrifices, which they slaughter” is interpreted as referring to offerings that I have previously permitted for you to slaughter on private altars. This verse teaches that those offerings may now be sacrificed only inside the Tabernacle. The phrase “upon the open field” tells you that in the case of one who slaughters an offering on a private altar during a period when the prohibition against sacrificing on private altars is in effect, even if he sacrifices the offering to God, the verse ascribes him blame as if he sacrificed it upon the open field in idolatrous worship.