you ignore all class, race and level requirements on the use of magic items

and

using your spell attack bonus

From p.207

Some spells require the caster to make an attack roll to determine whether the spell effect hits the intended target. Your attack bonus with a spell attack equals your spellcasting ability modifier + your proficiency bonus.

A spellcasting ability modifier is only given as a specific class ability. Barring multi-classing, a 13th level thief does not have a spellcasting ability modifier so this is 0. They have a proficiency bonus of +5. So +5. (DMG p.141)

Similarly for the saving throw.

Give it to the wizard!

This has been addressed in the latest Sage Advice.

Thank you, @V2Blast

Sage Advice

Does the Thief’s Use Magic Device feature allow them to use spell scrolls?

Yes. The intent is that a Thief can use spell scrolls with Use Magic Device.

DMG page 141 is also of some help here (Casting Spells):

The ability check is still required to cast from the scroll but, as a Rogue has no spell-casting ability score, their's is always treated as zero when casting spells from magic items. It would stand to reason that a check would be in order to cast the spell from a scroll as well, the only issue is that it is just a d20 roll for them.

If you don't have a spellcasting ability—perhaps you're a rogue with the Use Magic Device feature—your spellcasting ability modifier is +0 for the item...

It does go on to say that your proficiency bonus (PB) does apply but this is specifically called out as an ability check so your PB does not apply to those. If it were a Wand or Staff, is what I believe the PB is intended to use.