The idea of a composition in a single movement being nevertheless called a symphony was not new to Sibelius; in 1906 Schoenberg wrote his first Chamber Symphony in one movement, Franz Schreker following with his own Chamber Symphony in one movement in 1916, and Karol Szymanowski completing his one-movement Third Symphony also in 1916. The earliest (post-Haydn) symphony in multiple movements all played attacca, essentially forming one continuous movement, is of course Schumann's Fourth Symphony from 1841, initially titled "Symphonic Fantasia", where all four movements also share thematic material, but this kind of experimentation with symphonic form did not really take off until the 20th century; e.g. Nielsen's Fourth Symphony (1916) is in four linked movements sharing thematic material, Zemlinsky's Lyrische Symphonie (1923) in seven linked movements, Scriabin's Third Symphony "The Divine Poem" (1902) in three linked movements. And Scriabin's Fourth (1908) and Fifth Symphonies (1911) are both in one movement, although Scriabin himself dithered over whether to call them symphonies or tone poems.
Essentially this was a time period when the distinctions between symphonies and tone poems were beginning to erode, and composers were developing radically different conceptions of symphonic form; including applying the title to works for small chamber ensemble (as small as seven players in Gavriil Popov's Chamber Symphony of 1927) as well as works in one movement or multiple linked movements, works as long as two hours (Havergal Brian's First Symphony of 1927) or as short as ten minutes (Anton Webern's Symphony of 1928), et cetera. In this artistic context Sibelius's Seventh Symphony (1924) fits in perfectly: by the time it was completed, the symphony had transitioned into much more of an abstract description of character, rather than a concrete form per se.
In other words it's a symphony because he called it a symphony, but there is a good deal of historical context for why he did so.