I would say that it does. I will describe why with very colorful hyperbole's and circular logic, and all the things that make it difficult to find the information you are looking for.
When relying on descriptions 100% of the time to do 100% of your thinking for you:
If you relied on every single spell description to tell you that your spell indeed exists on your current plane of existence, few spells would exist at all. Or if you wanted to argue that this is more of a divine ability, then its still a spell, because those are spells too. (I would argue that this is a divine ability).
"R.S.B." is a spell attack and would benefit from spell sniper. I can see that other people have their way of "interpreting" the rules. They have valid points, but the only thing "very clear" about this is that its vague at best, like most other things. Now read my favorite line that never helps out the player. "It can be a D.M. ruling."
I believe in the song "don't stop believing" and I also believe "R.S.B." count's for spell sniper because:
"This special attack is a "ranged spell attack" with a range of 30 feet." A ranged spell attack counts with, you guessed it, spell attacks. If it was a non-spell like attack, you still would have the infamous non-magical "ranged attacks." It's like the division between fiction and non-fiction. Its either one or the other. Or both. Or neither?
If you look at the wording it just sounds like the author didn't want to create a whole spell, spell category, and meta for this "spell-like ability." When they can just use the blanket "special attack" for it. "R.S.B."really is unique. It's an at-will, holy damage, instant attack, that can be used multiple times in an action, and only monks can use it. That is a lot of clarification they need to do to put it neatly in their already established spell schema.
Lastly, I don't know why this has to be clarified but, Spell attacks are used on attack actions and bonus actions. You use your attack action and or bonus action to cast a spell. There is no such thing as a spell action.
Saying, "Radiant Sun Bolt is very specifically still the Attack action - it is not a spell." is non-sense. The spell says "When (you) take the (Attack action) (on your turn) (and use) this (special attack) (as part of it)" it is used exactly like every single action timed spell ever (in combat).
It very specifically and clearly says special attack during an attack action. Which is the exact scientific equivalent of an apathetic shoulder shrug, while eating a bland cheese sandwich, and saying "It's an attack... that's special..." There is a reason they started labeling things as "Special Melee Attack" now.