Not true. They are very similar identical especially Northern Tigray and southern Eritrea. Southern Tigray does have mix with Amhara and Agew but it’s not big enough.Well apartenyly from what I’m hearing they aren’t the same people they just speak the same language. Tigrinya speakers in Tigray are mostly agew or agew mixed whereas tigrinya speakers in eritrea are mostly beja or Beja mixed
the difference between them will be like the difference between 2 Somali qabils. While there is different history and identity if you took a DNA test itd come out nearly identical.