This isn't right either.
The SNP Constitution doesn't directly address candidate eligibility; it charges the NEC with setting out rules for each election, in line with Standing Orders and driven by the membership via the National Conference.
In August 2020, the NEC set rules for the Holyrood elections of May 2021. This introduced the requirement for sitting MPs to resign before standing as MSPs. SNP MPs weren't prevented from standing, but it did become significantly more complicated and most who had expressed an interest before the change abandoned their plans to avoid closing their constituency offices and dismissing their staff. A group of SNP parliamentarians wrote to the FM protesting the policy, to no avail. There was also a court case by an SNP activist that went nowhere.
The rule absolutely was binding for the last Holyrood election. Here's Neil Grey, the only SNP MP who actually did stand as an SNP MSP (although two other SNP MPs switched party and stood) explaining why he had to resign. It's ultimately up to the NEC to keep, change, or discard this requirement when the rules for the next Holyrood election are set.