Explanation
DHCPOFFER is used to identify spurious DHCP servers. A spurious DHCP server is any device that is configured to act as a DHCP server without the network administrator's knowledge or permission. A spurious DHCP server can cause network problems by assigning incorrect or duplicate IP addresses to clients, or by redirecting traffic to malicious gateways. To prevent such attacks, the DHCP snooping feature can be enabled on switches to filter out invalid or unauthorized DHCP messages from untrusted sources1.
DHCP snooping works by intercepting and validating DHCP messages on a per-VLAN basis. The switch maintains a DHCP snooping binding database that contains information about the trusted hosts with leased IP addresses, such as MAC address, IP address, lease time, binding type, VLAN number, and interface information2. The switch also classifies its ports as trusted or untrusted. Trusted ports are those that connect to authorized DHCP servers or other trusted switches. Untrusted ports are those that connect to untrusted hosts or devices. The switch only allows DHCP messages from trusted ports, and drops any DHCP messages from untrusted ports that do not match the information in the binding database3.
The switch uses DHCPOFFER messages to identify spurious DHCP servers. A DHCPOFFER message is a response from a DHCP server to a client's request for an IP address. The message contains the offered IP address, subnet mask, default gateway, and other configuration parameters for the client4. When the switch receives a DHCPOFFER message from an untrusted port, it compares the source MAC address and the offered IP address with the binding database. If there is no match, the switch considers the message as coming from a spurious DHCP server and drops it. The switch also logs an error message and increments a counter for the number of dropped messages5.
References:
1: Configuring DHCP Snooping - Cisco
2: Catalyst 6500 Release 12.2SX Software Configuration Guide - DHCP Snooping Binding Database
3: What is DHCP Snooping? - IONOS
4: Dynamic Host Configuration Protocol (DHCP) and Bootstrap Protocol (BOOTP) Parameters
5: Configuring DHCP Snooping - Cisco

Explanation
The trunk still forms with mismatched native VLANs and the traffic can actually flow between mismatched switches. But it is absolutely necessary that the native VLANs on both ends of a trunk link match; otherwise a native VLAN mismatch occurs, causing the two VLANs to effectively merge.
For example with the above configuration, SW1 would send untagged frames for VLAN 999. SW2 receives them but would think they are for VLAN 99 so we can say these two VLANs are merged.