[alxlaz]

Most crystals have better sealing :-). SiTime explicitly(-ish) acknowledges that their older-generation packaging was less efficient on their website:

"How effective is the hermetic seal of MEMS oscillators??

One of the key elements enabling extremely stable MEMS resonators is SiTime’s EpiSeal™ process which hermetically seals the resonators during wafer processing, eliminating any need for hermetically sealed ceramic packaging. SiTime’s EpiSeal resonator is impervious to the highest concentration elements in the atmosphere, nitrogen and oxygen, and therefore acts as a perfect seal. Previous generations of EpiSeal resonators may have been impacted by large concentrations of small-molecule gas. Newer EpiSeal resonators are impervious to all small-molecule gases. Please contact SiTime in case you are planning to use a SiTime device in large concentrations of small-molecule gas, so that we can recommend an appropriate, immune part."

https://www.sitime.com/support/faqs

[NormM]

Apple uses these oscillators. They are substantially more expensive than crystal oscillators, but are much smaller and use less power. This may explain why the Android phones, which are more price sensitive, didn't use them. It does seem that the current generation of SiTime oscillators don't have a problem with Helium, so perhaps current generation iPhones don't either.