In NYC a rental apartment is usually leased for a 12 month term, 24 months at the most. Purchasing a coop is permanent. A guarantor for 1 years rent is quite different than a guarantor for life.
Coops that allow guarantors or co-purchasers will want the guarantor to guarantee maintenance payments for ever.
However, I just checked Insurent's website because I was intrigued by your question. The site does say it is a guarantor for coops and condos owners. In my opinion that means it guarantees rent payment to coop and condo owners that sublet their apartments.
It would be up to each individual coop as to whether they would accept Insurent as a guarantor. Coop's have their own discretion regarding approving or rejecting a prospective purchase.
It is certainly an interesting concept since many potential renters and buyers in NYC do not have a wealthy family member that can be a guarantor or co-purchaser.
Lic Assoc RE Broker
The Corcoran Group