Source (in Dutch): https://www.telegraaf.nl/sport/autosport/red-bull-raakt-rechterhand-max-verstappen-kwijt-gianpiero-lambiase-gaat-in-op-astronomisch-aanbod-van-mclaren/145448070.html
Via Deepl:
Lambiase (45) is under contract with Red Bull Racing through 2027. Until then, the English engineer—who also serves as Head of Racing at Red Bull—will not be going anywhere. He is expected to join rival team McLaren in 2028. This has been confirmed by multiple sources following earlier reports by De Limburger.
By bringing in Lambiase—scheduled for 2028—McLaren is already preparing for the possible departure of Andrea Stella. The current team principal is well-regarded internally at the Woking-based team, but has been linked with a return to Ferrari. For Lambiase, therefore, a significant promotion—both financially and in terms of position—lies ahead.
Or he knows Max is no longer in the picture and wants to be with a winning constructor and driver lineup.
Or he receives a fuckton of money
Multi-million pounds, apparently.