The blanket claim is too broad

FTC says manufacturers generally cannot require specified parts or services to preserve warranty coverage unless provided free or covered by an FTC waiver. Keep the written warranty rather than relying on a sales explanation.

Sources[1] FTC

Damage from a repair is a separate issue

FTC's vehicle guidance says outside routine maintenance alone cannot justify denial. But if a defective part or poor installation causes damage, coverage for that damage can be denied if the manufacturer or dealer proves the connection.

Sources[2] FTC

Keep the evidence and identify the contract

Save maintenance receipts and ask for a written reason for refusal. Compare separately purchased service contracts with the included warranty: overlapping coverage, deductibles and approved repair facilities can change the value.