Ostkaka was made all over Sweden, also in Denmark, Norway, the Netherlands etc. The recipe varied a little but the basic ingredients are milk (into which rennet is added - rennet makes the milk coagulate which is why it's used for making cheese, giving the cake its name although there's no cheese in it) and eggs, making it the ultimate rural luxury food.
Traditionally, both milk - particularly in its preserved forms (butter and cheese) - and eggs were sold, or even used directly to pay e.g. taxes, and consumed only in small amounts by the farmers/producers themselves. To wantonly use lots of milk, cream and eggs on yourself, your family and your guests was therefore a form of conspicuous over-consumption, showing off. Dishes made from fresh milk/cream and eggs were also traditionally served to new mothers; ostkaka was one such dish but there's also e.g. äggost (basically the same thing but no rennet is added, instead it's boiled until it splits).
Ostkaka was traditionally served at all major parties; it was e.g. mandatory at weddings. It was however not commonly part of traditional Christmas meals - or at least, it wasn't in cities. It may have been served in the country, making it even more of a luxury item because of the scarcity of the main ingredients in winter.
Preserving food traditions in a new country usually concentrates the traditional dishes to festive family occasions - and what is more festive and family-oriented than Christmas? - even though a particular dish may never - or only rarely - have been served at a particular feast in the old country. Getting the old dishes together can be difficult in a new country and so all traditional dishes tend to be served only once or twice a year, as a sort of all-out effort. Also, usually on "family-only" occasions where everybody (more or less) comes from the same country; eating a traditional national dish re-inforces family ties (and ensures that you get nice reviews for your efforts).
Today the Småland ostkaka is certainly the most famous one, but that probably says more about modern consumption habits than anything else - there's a dairy in Småland (Frödinge Mejeri) that makes an ostkaka sold in supermarkets all over Sweden. Just saying "ostkaka" more or less conjures up an image of the Frödinge ostkaka box rather than of all the versions ostkaka used to be made in. So instead of making your own, local and labour-intensive version people pick up a Frödinge (=Småland) ostkaka (comes in several sizes) and briefly heats it at home. But there are some local pockets of resistance - in e.g. Hälsingland they stick to their own version (a very traditional province, Hälsingland), even arranging competitions etc., and thus making the Hälsingland ostkaka the next most famous in Sweden.
Traditionally a Småland ostkaka today has almonds in it, but this may not have been the case in the 19th century since almonds have always been imported and thus were very, very expensive.
So, to conclude: * Ostkaka may have been served at rural Christmas parties in the old times, but if it was it hasn't made it into modern Swedish Christmas traditions - only these change, e.g. in my childhood salmon was never part of the julbord (Christmas smorgasbord) but is today a staple. * Your ostkaka recipe may have come from either Östergötland or Småland (remember, they are neighbouring provinces). Today's use of almond in a Småland ostkaka isn't proof that it was used in rural Småland a century or more ago. Or the almond disappeared in the Atlantic.
BTW, "lutefisk" is Norwegian (I know, this is the version that made it into American English) while in Swedish it's "lutfisk" (no "e"). Personally, I won't touch it but it was of course mandatory in my childhood (no lutfisk - no presents).