Under about 15kg, the lateral suture remains a legitimate, evidence supported choice at roughly half the cost of TPLO, and many small dogs do excellently with it. TPLO gives a more consistently athletic result and is increasingly recommended for active small dogs too, but for the average small companion dog the suture is not a second class option.
The honest decision factors: the dog's activity level, the surgeon's experience with each technique in small patients, and money. A very active 12kg terrier that lives to run has a stronger TPLO case than a 6kg lap dog. What does not decide it: a clinic that only offers one technique presenting that technique as the only option.
If you have one quote for a $6,000 TPLO and someone mentions the suture exists, that is precisely the moment a $250 independent read of the x-rays pays for itself.
Send what your vet said and quoted. A veterinary surgeon reads it and comes back to you with what they can do and what it would cost, before you pay anything. A written second opinion is from $250.
Send the case, hear back from a surgeon with what they can do and what it costs, and only pay if you go ahead.