RAW: Yes, the item can be used to affect Rakshasa
Disclaimer: As always, talk to your DM about this. Rulings aren't always black and white, and while the information below tries to be as objective as possible, it's impossible to remove all ambiguity and it's well within reason that a DM would rule otherwise.
As stated in the question, there are two different types of magic one can use; Spells and other Magical Effects. We can prove that these are two discrete keywords that aren't always mutually present by looking at other magic items such as the Mantle of Spell Resistance [DMG p. 180] that only applies to spells whereas items such as the Robes of the Archmagi [DMG p. 194] has an ability that applies to both spells and magical effects. We just need to determine which one the Artificer's ability falls under.
To figure out where it falls under we just need to look at the wording of other magical items. In this case we'll look at wands, as they align the most with the effects generated from this class ability. Wand of Fireballs [DMG p. 210] states. (emphasis mine)
This wand has 7 charges. While holding it, you can use an action to
expend 1 or more of its charges to cast the fireball spell (save
DC 15) from it. For 1 charge, you cast the 3rd-level version of the
spell. You can increase the spell slot level by one for each
additional charge you expend.
Similar wording is found on the Wand of Magic Detection [DMG p. 211], Staff of the Woodlands [DMG p. 204], Sending Stones [DMG p. 199], among many, many others. We also see that some items produce effects similar to that of spells without using the keyword cast. While no such items exist (as of the DMG) that exactly duplicate every aspect of a spell, we do get close with the Plate Armor of Etherealness [DMG p. 185] which differs only in duration. (emphasis mine)
While you're wearing this armor, you can speak its command word as an
action to gain the effect of the etherealness spell, which last for 10
minutes or until you remove the armor or use an action to speak the
command word again. This property of the armor can't be used again
until the next dawn.
Other notable items are the Wand of Fear [DMG p. 210], Censer of Controlling Air Elementals [DMG p. 158], and arguably the Necklace of Fireballs [DMG p. 182]. Given that the book references the other items in a consistent manner, we can gather that this is a deliberate choice and while functionally similar to the etherealness spell it is, in fact, its own magical effect altogether.
Now that's for magic items, but what about class abilities? We can see the same keywords persist across class abilities by looking at the Warlock's Mystic Arcanum ability [PH p. 108] which once again uses the cast keyword similar to magic items.
Looking back on the Artificer class ability, we see that the cast keyword isn't present at all. The creature produces an effect from the object, similar to how the Wand of Fear produces an effect imitating Command. The Artificer doesn't even cast the spell into the object as part of the creation process, it just gets stored. The item itself isn't even magical, according to a string of tweets by Jeremy Crawford here. (most relevant information below)
The D&D artificer's Spell-Storing Item feature doesn't turn an object
into a magic item. [...]
[...] Use an Object works only with nonmagical items, as clarified in the
DMG.
If the item of Spell-Storing Item is nonmagical, it's usable with Use
an Object.
In the end, it would be a magical effect subject to the Rakshasa's advantage on saving throws and nothing more.
