According to EOI-Tax (link below), a website run by the OECD, Guatemala has signed TIEAs with Costa Rica, Denmark, El Salvador, Faeroe Islands, Finland, Greenland, Honduras, Iceland, Nicaragua, Norway, and Sweden.
Exchange of Information Portal
Claiming that Guatemala has not signed any TIEAs is incorrect. It has signed multiple. However, it is debatable how effective these are until the Guatemalan government updates its laws. The OECD even acknowledges this its peer review from earlier this year.
"However, due to restrictions in Guatemala’s domestic law, it has no agreements that provide for effective exchange of information with its partners. The tax authority does not have adequate powers to access information and ownership information is not always available for foreign companies, foreign partnerships or foreign trusts active in Guatemala."
Guatemala:- Tax evasion: Pressure to end tax evasion grows as the Global Forum publishes new reviews - Organisation for Economic Co-operation and Development
It is worth noting that this peer review was written and published before Guatemala signed this year's round of TIEAs. I can only imagine that Deloitte's report, too, was written before these signings took place. Next year's reports will hopefully shed some light on the matter.