Just registered on Lemmy at last to pitch in! "I've just xyz" is much more common in most Englishes than "I just xyz", because the present perfect tense implies some connection to the present, hence "present perfect", and is perfectly correct English. The author has simply omitted "I've", which is common in colloquial speech. This is also common in Dutch, a closely related language that I speak every day as a second language, if that helps legitimise it for anyone: "net gezien" as shorthand for "ik heb net gezien". In fact, while there are a number of problems with the post, none of them are ("none of them is" for the pedants) grammatical. I assume from the English in your post that it's not your first language so hopefully this is more helpful than annoying.