Ambiguous Syria

I'm continuing my way through Louis E. Newman's The Sanctity of the Seventh Year: A Study of Mishnah Tractate Shebiit. In this post, I'll quote from page 121, which is about different positions in Mishnah Tractate Shebiit about the status of Syria:

"[Syria] poses a problem for the application of the [seventh year land rest] law, because it is not within the boundaries of the Land, yet many Israelites settled there in the period following the Babylonian exile. It thus is not clear whether Syria is subject to the same rules as the Land of Israel. Three distinct views are put forward in the chapter. Aqiba (M. 6:2C-E) and Simeon (M. 6:5B, D) hold that the extensive Israelite settlement within Syria has, in effect, made it part of the Land. Accordingly, all the restrictions that apply in the Land of Israel are also in force in Syria. At M. 6:2A-B, Mishnah's authorities claim that Syria is subject only to some of the restrictions of the Sabbatical year, for it was settled only relatively late in Israelite history. Finally, M. 6:5-6A, C treats Syria as a foreign country. Since it is outside the original borders of the Land, it is entirely exempt from the restrictions that apply there."

I do not understand why Syria was not considered to be part of the Promised Land. In my post here, I said the following about the different forms of the land promise in the Hebrew Bible:

"A tradition says that God desired for Israel’s northernmost boundary to be up to the gate of Hamath in Syria (Numbers 34:8), which was the case under King Solomon (I Kings 8:65); in another tradition, however, Israel’s northernmost boundary was to be further north than the gate of Hamath, extending all the way to the Euphrates River (Genesis 15:18), as was true under Solomon (I Kings 4:21, 24). And so there are different traditions about what God wanted Israel’s boundaries to be, and where they were under King Solomon."

So why wouldn't Syria be a part of the Promised Land? Hamath (which was to be the northernmost border, according to Numbers 34:8) was in Syria, and the Euphrates (which Genesis 15:18 prescribes as the northernmost border) is further north than Syria, meaning that the Promised Land in that scenario would include Syria. For the rabbis, is it technically not Israelite land because the Israelites did not have a tradition of inhabiting it, since Syria for a long time was its own country---more than that, it was an enemy to Israel?

The notion that the Israelites could make Syria part of the holy land (on some level) by dwelling there after the exile is interesting, but there has to be more to that. After all, Jews inhabited countries in the Diaspora, but (as far as I know) they didn't make those part of the holy land by dwelling in them. What makes Syria different? Its vicinity to Palestine? The fact that it is technically included in the land promise? I checked the discussion in M. Shebiit 6, and, as far as I could tell, it did not address these sorts of questions.