In the context of the preceding posukim, Bereshit 35:10-12, this angel confirms the elevation of the status of Yaacov Avinu to Yisrael, like was mentioned in connection with the other angel (the Angel of Death) with whom he wrestled in Bereshit 32:29.
Additionally, he is told that G-d has decided that he and his descendants will be the inheritors of the blessing given to Avraham Avinu and Yitzchok Avinu, namely of the land.
And this is in fact, what Rashi points to and what the Siftei Chachamim to Rashi on Bereshit 35:14 clarify. Rashi is not discussing with his comment, "I do not know what this teaches us.", the pillar at all. He is pointing out that the words, ‘where He spoke with him’ are redundant, meaning extra words, that are pointing to Avraham & Yitzchok & the covenant made there. The exact same expression "בַּמָּק֛וֹם אֲשֶׁר־דִּבֶּ֥ר אִתּ֖וֹ" appears in the preceding posuk, Bereshit 35:13. Rashi is saying that he has no tradition about what this redundant reference is teaching aside from that it is pointing to Avraham and Yitzchok.
This blessing of possessing the land was accomplished by Avraham and Yitzchok through the Akeida, which was a type of spiritual service that involved subjugating the Angel of Death via the ram that was sacrificed.
This paradigm, which was established at the Akeida, is what is being done via a type of prayer by Yisrael Avinu in Bereshit 35:10.
The word matzevah (מצבה) comes from the root יצב, which has a connotation of a type of prayer as referenced by Jastrow on the root quoting Pesikta Rabbati which is referencing Mishlei 22:29. (See also Shir HaShirim Rabbah 1:1:1 and Shemot Rabbah 11:1)
The stone mentioned here is referring to a letter being removed from the name of the Angel of Death. The letters of the Aleph-Beit are called stones in Sefer Yetzirah (4:16 Gra edition) which tradition teaches was given to us via Avraham Avinu.
This removal of that single letter (the letter Mem, מ) from the name of the Angel of Death converts the severity (harsh judgement אלהים) associated with the Angel of Death into softened judgement (אכדטם). This type of letter transformation, converting the letters to the letter preceding it in alphabetical order, is the transformation associated with Avraham Avinu.
So for example, when you see יהוה in the Amidah prayer connected to Avraham Avinu (Elohei Avraham), you intend טדהד. The transformation associated with Yitzchok Avinu is the opposite, meaning letters transform into the letter following it in alphabetical order. So יהוה becomes כוזו. The transformation for Yaacov Avinu, Yisrael, is the At-Bash. So יהוה becomes מצפץ.
This removal of that single letter from the name of the Angel of Death also converts the name of the Angel of Death into one of the 3 letter triplets (ס״אל) from G-d's 72 triplet-letter name (It contains 72, three letter roots for a total of 216 letters.) That particular triplet is associated with G-d's Kingship like is explained by Rabbi Yaacov Tzvi Yolles in Sefer Kehillat Yaacov. This concept is also referenced in the name of the Ba'al Shem Tov in Sefer Keter Shem Tov. It's also worth noting that if you apply the letter transformation of Yaacov Avinu, meaning the At-Bash, to the three letters remaining from the Angel of Deaath's name, the result is the Angelic name referenced in the Talmud for the posuk from Ashrei that is associated with our daily parnussa (Tehillim 145:16).
The harsh judgement is characterized by the red wine being poured out on the stone (Red being associated with Gevurah/Severity.). And the conversion to softened judgement is alluded to via the oil which follows.
Shemen (שמן) is referring to (שם-נו״ן), that G-d's 6 letter name which is the union of אהיה and יהוה (these two names have a gematria sum of 50 when including the two names and kollel). But their union (אהיהוה) only has a gematria of 32. When including the kollel, the resulting sum is 33 (ג״ל or ל״ג, which is also the value of כד״ט from the softened judgement of אכדטם mentioned above) as in Lag b'Omer, the time between Passover and Shavuot when the decree on the students of Rabbi Akiva stopped. The harsh judgement was overturned.
And this paradigm associated to 33, meaning Gal (גל) and the overturning of harsh judgement is repeated between Yaacov and Lavan at the Gal Eid (גל עד) and also with Moshe Rabbeinu and Pinchas in connection with the spear (הרמח, the Mem is from the Angel of Death and פינחס, like יצחק is gematria 208, ר״ח) used against Zimri and Kozbi.
So what is being described in connection with Bereshit 35:14 is not a violation of the prohibition you quote from Devarim 16:22, but an affirmation of the paradigm established by Avraham Avinu at the binding of Yitzchok.
