It is and it isn't contagious. From what I could gather when my betta had it, it is contagious when the lump splits and releases spores.
People of a sensitive disposition, stop here
I bought the betta on line as I couldn't find any plakats (short fins) in the shops. I put him in my 50 litre tank with a shoal of pygmy cories. Within a few days he looked as though he'd caught a scale but instead of getting better it stayed there. Then it started growing and I realised it was a lump. Because dragonscale bettas have very thick scales, it never showed as white. After several weeks I decided something wasn't right and google found lymphocystis so I set up the QT and moved him there as I still had another betta in the betta tank, though he died of old age shortly afterwards. The lump continued growing and every now and then it sort of burst and showed a grey gooey mass in the sunken crater of a lump. I presume this is when spores get released. I had a separate siphon tube and bucket for this tank and I never used anything in this tank in another. Eventually, he grew several lumps down the same side and the first one, which was just above his gill, got so big it moved into the gill itself. That's when he went downhill, refusing to eat, sitting in a plant all day, so I put him down. Then sterilised the tank, filter, heater etc, threw the plastic plants and filter media in the bin.
He was in the same tank as the pygmy cories for a few weeks, but I'd moved him out before the first time the lump split. The cories never caught it.
Attached - a couple of pics taken shortly before the end.