Can SuiteQL be used to insert or update records?

Nop, read-only

As a general rule for ERP databases, direct SQL write operations would be a big no. A write operation is typically not a single operation - it consists of multiple touch points at the business layer. Most direct writes would disrupt or corrupt intrinsic referential integrity between tables. Having said that, in the old days when databases were still on-prem and admins had full access, you could update or overwrite a single field if you were 110% sure that it would not impact anything else. 🙂

I don't disagree with Emanuel, but I haven't seen any evidence that SuiteQL is directly tied to the database schema - indeed 'writes' using SQL in this context may not actually be executed directly on the database, and almost surely not directly on the tables. It is possible that SuiteQL insert/updates could interact with some high level business layer which does the needful.