Yes, it is true. You can see in the tweet thread that Silliman cites a source from an academic press, and in looking into it, the book checks out. I can completely understand why you would question it, though! Twitter history that goes viral is very often off-base or significantly flawed. (DON'T TALK TO ME ABOUT BEAU BRUMMELL.)

In the northern and southern English colonies of North America that would eventually become the United States, pumpkin (or "pompion", as it was sometimes called) was widely cultivated because it grew so well with little tending and could be used in so many ways: brewed into beer, boiled or roasted as a vegetable, mashed into thick soup or pancake/quick bread batter, and made into pies, tarts, and puddings; it could even be processed into a naturally sweet paste as a replacement for expensive, imported sugar. It also was ready for harvest after the other crops and kept well even without being preserved. In the colonial era, the pumpkin was really more a food of necessity than a prized crop - something you grew just to get by, if you couldn't afford cider, meat, sugar, etc. - but by the days of the early republic, it had taken a space in the national imagination as an important part of a specifically American cuisine. The first really American cookbook, Amelia Simmons's American Cookery (1796), included two recipes for pumpkin pie, reflecting its importance: one richer and made with sugar (indicating its use for the table in a wealthier household that could in theory have afforded something else), the other made with molasses and fewer spices.

Eating pumpkins signified a family’s taking care of itself on its own piece of land, no matter how humble the size, and depending on no one but itself. Growing pumpkins required nothing more than land, honest work, and family. Because the pumpkin produced large yields on minimal plots, wealth and social class were superfluous to its production. Any man, at least any white man, could plant pumpkins, and every woman could transform them into food for the family, nourishing themselves and the nation’s democratic aspirations in the process. The pumpkin, therefore, made a strong political statement. ... Yet while some Europeans scorned the pumpkin as rural peasant food and others equated it with hedonism, the vegetable triggered a sense of pride and nostalgia in some early Americans. It did more than fill dinner plates; it communicated a set of assumptions about who Americans were and what America stood for in contrast to Europeans.

(Cindy Ott, Pumpkin: The Curious History of an American Icon)

At the same time, the increasing prosperity and industrialization of the new nation meant that fewer people were actually eating pumpkin as a regular dish - they could afford winter squashes and proper ale, and could relegate the pumpkin to livestock feed if they even bothered to cultivate it at all. In fact, the main distinguishing feature of a squash versus a pumpkin was that people wanted to eat it, in general! To the urbanizing population, the pumpkin was a symbol of rural life and the negative stereotypes of backwardness that went along with it.

The exception to all this was pumpkin pie in New England, so tied to regional identity that many cookbooks called it specifically "New England pumpkin pie". Despite not needing to use pumpkin as a sugar substitute or as a mashed vegetable on the table, New Englanders were very happy to use it as a pie filling. This is probably because those rural stereotypes were (and to some extent still are) held proudly as self-identifiers in the northeast, in a way that they weren't in the south. Ott also ties it to parallel feelings about unrefined nature vs. artificiality, which likewise connected to New England's self-fashioning as rugged, unpretentious individualists. The new-ish holiday of Thanksgiving in particular tied into this view, since much of the way it is and has been celebrated tied into harvest festivities, with a large dinner featuring seasonal domestic produce as a central focus. Spread-out extended families in New England would traditionally (or stereotypically) go back to the farmhouses of their parents and grandparents to enjoy the meal, which would often conclude with the traditional pumpkin pie.

The pie began to tie in with abolition in the decades leading up to the war, as the magazine writers like Sarah Josepha Hale (later to become most famed for her work on Godey's Lady's Book) who promoted Thanksgiving as a national holiday - which it wasn't at the time - were also writing and publishing material against slavery. I will allow Ott's clear prose to draw the connections again:

The writers interpreted agrarian ways of life or modes of production in social and political terms, imagining the small family farm as an antidote to the plantation. Rather than being determined to spread the agrarian way of life itself, they sought to spread the values and sense of morality they saw embedded in it and in themselves. ... Whittier, Child, and Hale, some of the most outspoken abolitionists, also wrote some of the period’s most popular and cherished pumpkin stories. To them, the pumpkin—and pumpkin farming—so completely embodied New England, as opposed to southern, values. The pumpkin was a naturally abundant crop that exemplified agrarian prowess, yet at the same time it was an unmarketable crop that stood for timeless agrarian values uncorrupted by the pursuit of profit.